I use several approaches to calculate this
First --
1. Only those who identify as Christians will be raptured and worldwide 31% of the world's population identifies as Christian.
John 14:6 Jesus saith unto him, I am the way, the truth, and the life: no man cometh unto the Father, but by me.
2. Statistics indicate that 30% of those who identify as Christians go to church each week. This brings the total down to 9% of the world's population.
Hebrews 10:25 Not forsaking the assembling of ourselves together, as the manner of some is; but exhorting one another: and so much the more, as ye see the day approaching.
3. Of those who are in church the Lord indicates half will be cut asunder. (However, to eat and drink with the drunken could indicate they don't attend church regularly, could be those who come at Christmas and Easter only). Therefore I would conclude this could be referring to the 21% of people identifying as Christians but who don't attend church. So the range at this point is 4.5% to 9%.
45 Who then is a faithful and wise servant, whom his lord hath made ruler over his household, to give them meat in due season? 46 Blessed is that servant, whom his lord when he cometh shall find so doing. 47 Verily I say unto you, That he shall make him ruler over all his goods. 48 But and if that evil servant shall say in his heart, My lord delayeth his coming; 49 And shall begin to smite his fellowservants, and to eat and drink with the drunken; 50 The lord of that servant shall come in a day when he looketh not for him, and in an hour that he is not aware of, 51 And shall cut him asunder, and appoint him his portion with the hypocrites: there shall be weeping and gnashing of teeth.
Here they are both servants and they are both in the house and they both refer to Jesus as "my Lord". Jesus calls these ones "evil".
4. Then we have the ten virgins. None of these are evil, but half of them are foolish.
25:1 Then shall the kingdom of heaven be likened unto ten virgins, which took their lamps, and went forth to meet the bridegroom. 2 And five of them were wise, and five were foolish. 3 They that were foolish took their lamps, and took no oil with them: 4 But the wise took oil in their vessels with their lamps. 5 While the bridegroom tarried, they all slumbered and slept. 6 And at midnight there was a cry made, Behold, the bridegroom cometh; go ye out to meet him. 7 Then all those virgins arose, and trimmed their lamps. 8 And the foolish said unto the wise, Give us of your oil; for our lamps are gone out. 9 But the wise answered, saying, Not so; lest there be not enough for us and you: but go ye rather to them that sell, and buy for yourselves. 10 And while they went to buy, the bridegroom came; and they that were ready went in with him to the marriage: and the door was shut. 11 Afterward came also the other virgins, saying, Lord, Lord, open to us. 12 But he answered and said, Verily I say unto you, I know you not. 13 Watch therefore, for ye know neither the day nor the hour wherein the Son of man cometh.
They are all going out to meet the Bridegroom, they all have lamps that are lit, but the foolish ones don't have oil with their lamps, they depend on borrowing oil from others. These are those who go to church, they aren't evil, but they depend on the Pastor to give them oil each week rather than having their own experiences of the Lord. The foolish ones are shut out and told to go buy oil from those who sell it. They will have to pay the price for oil during the tribulation and will be the tribulation saints. This cuts the number to a range of 3-4.5%.
5. Here the Lord refers to the "unprofitable" servant. Not evil and not foolish.
14 For the kingdom of heaven is as a man travelling into a far country, who called his own servants, and delivered unto them his goods. 15 And unto one he gave five talents, to another two, and to another one; to every man according to his several ability; and straightway took his journey. 16 Then he that had received the five talents went and traded with the same, and made them other five talents. 17 And likewise he that had received two, he also gained other two. 18 But he that had received one went and digged in the earth, and hid his lord's money. 19 After a long time the lord of those servants cometh, and reckoneth with them. 20 And so he that had received five talents came and brought other five talents, saying, Lord, thou deliveredst unto me five talents: behold, I have gained beside them five talents more. 21 His lord said unto him, Well done, thou good and faithful servant: thou hast been faithful over a few things, I will make thee ruler over many things: enter thou into the joy of thy lord. 22 He also that had received two talents came and said, Lord, thou deliveredst unto me two talents: behold, I have gained two other talents beside them. 23 His lord said unto him, Well done, good and faithful servant; thou hast been faithful over a few things, I will make thee ruler over many things: enter thou into the joy of thy lord. 24 Then he which had received the one talent came and said, Lord, I knew thee that thou art an hard man, reaping where thou hast not sown, and gathering where thou hast not strawed: 25 And I was afraid, and went and hid thy talent in the earth: lo, there thou hast that is thine. 26 His lord answered and said unto him, Thou wicked and slothful servant, thou knewest that I reap where I sowed not, and gather where I have not strawed: 27 Thou oughtest therefore to have put my money to the exchangers, and then at my coming I should have received mine own with usury. 28 Take therefore the talent from him, and give it unto him which hath ten talents. 29 For unto every one that hath shall be given, and he shall have abundance: but from him that hath not shall be taken away even that which he hath. 30 And cast ye the unprofitable servant into outer darkness: there shall be weeping and gnashing of teeth.
These three are involved in "ministry". Two thirds of them are actively involved in ministry and the other one buries the talent in the backyard. So you have Christians who attend church regularly, they are not evil, they are not foolish, but they are slothful and then make excuses for being slothful by blaming the Lord. Hence the Lord calls them wicked. This drops the estimate to 2-3%. That is still 240 million people. A great multitude which no man could number in Revelation 7.
This is one way to calculate it.
Second --
Joshua and Caleb give a good report, the other ten spies give an evil report. As a result all those who listened to the evil report must go through the wilderness and die in the wilderness. I see that as an example of 5/6ths of genuine Christians going through the tribulation. If 9% are genuine Christians then only 1.5% will be raptured prior to the tribulation.