As I see it, OT tithing was under the law. As stated, the NT does not set a specific figure. Most church leaders however will encourage the ten percent tithing as it places somewhat of a guilt trip on some and helps to ensure a steady income.
The problem with someone having an income of 40 million and tithing ten percent is that once four million has been recognized by the rest of the church membership..., the rest would quit tithing. What would be the point in tithing at all if the local church's check balance reflected a four million dollar balance? Then, here comes the envy, and the gossip, and the resentment. Having an income of millions would turn into a curse.
Those who then would NOT tithe because there's plenty of money in the bank would end up losing any and all blessings from the Lord from their tithing. When ones tithes above and beyond with a pure heart the Lord blesses above and beyond.
We would all like to have millions in the bank but just consider for a moment what it would do to our humbleness.
Did cain/Able bring the Lord first fruits/ a Tithe?
Did Abraham pay a tithe to Melchizedek?
Werent they before the law was given to Moses, so why was it under the law only?