Heart is important, no doubt about that.
But if you feel the % does not matter, what about what Paul stated about sow sparingly shall reap sparingly in 2 Cor 9:6?
The motive of the heart is always the deciding factor, the percentage given is only a number it is data ones and zeros. The actual tithing or giving is not to be given with that kind of mindset, it is to be given by how God leads.
To tithe or not to tithe is not the question to be asking and the percentage to give is not the question either the answer however is spirit lead. The women who gave what little she had did so because of how she loved and honored the Lord, that is a beautiful scenery in of itself but Jesus knew her heart he knew why she was giving in the first place if she had done that with a different heart and motive Jesus would have rebuked her too.
In my opinion there no justifying or not justifying tithing or the amount but rather why you are tithing.
I heard a story a good while ago about a aman who worked at the church and collected donations and came to an elderly man and his grandchildren and wife the old man had his money in hand but the collector noticed it was folded oddly but didn't think much of it.
When counting the collections he picked up the folded money and was shocked because the old man had folded it in a way where it looked like a 10 dollar bill but was actually a 100 dollar bill. This is what I mean, this old man gave a huge amount but was so humble and modest he folded it to look like it wasn't he never asked for or wanted anything from his deed.