I like Sander's proposed tax rate for $1 - $18,450: 10%
It can be improved though...here's how...$1 and up: 10%
Of all the people you, as a Christian should support a lower and a flat tax rate. Why? Simply because the less money you you give in taxes, the more money you have in your pocket.
And since you are a believer, then you are a generous person, unlike the other greedy people...which means, now you have more money to give to the Church and help the Church help the poor and needy as you want to.
Sure, the greedy world will have more money in its pockets too, and it'll do whatever it does with it. However, that shouldn't be your concern, right? You're always wanting to follow the Bible and live according to God's word. And whatever money you do get in your pocket, you do the best with it to help the Church. And even though the greedy may cheat on their taxes, you don't...because you want to "give to Cesar what is Cesar's".
So regardless of your income, if Cesar taxes you 10%, you will have the rest 90% to cover your expenses, and for doing God's work. And if your Christian brother/sister makes $40K, or $400K, or $4 million+ yearly, imagine how much more he/she can give (with his 90%) to the Church in obedience and with a cheerful heart.
Now, if part of your tax money is currently going out to help the poor (welfare), do you let it go as a cheerful giver? Or if part of your tax money that goes out to help the poor includes promoting non-Christian ideals, do you still want the government...instead of the Church...to decide how to use your money for welfare? Probably not.
So, yeah, its better if you and your wealthier and poorer Christian brothers and sisters have a flat tax rate...and even better if its low. Because this way you and they can give more to the Church both obediently and cheerfully.