Here's how I reason it to be based on gross income-
Point #1- The definition of the word "tithe" is "a tenth". There's no way around it, so you can't say it's any amount you feel like giving.
Point #2- The tithe began with Abraham, who operated under the Law of Faith.
Point #3- Since any taxes you pay are coming back to you in some form of service, e.g. public roadways, utilities, parks, military protection, police, etc., then you ought to pay tithes on your gross income. Whether you feel your getting a good bang for the buck is irrelevant. That's the price of living free.. Wait, that didn't seem to come out right!
Anyway, you get the picture.
Point #4- Is God greater than your government? I think most would agree yes. Same is true with health insurance premiums deducted from your paycheck. God is surely greater than your health care provider. Therefore, God should get the first cut. Jacob said, "and of all that thou shalt give me I will surely give the tenth unto thee." (Genesis 28:23).
Furthermore, Numbers 18:21 says, "And, behold, I have given the children of Levi all the tenth in Israel for an inheritance, for their service which they serve, even the service of the tabernacle of the congregation."
The word of emphasis here is "all".
Point #5- The most notable scripture on tithes and offerings is in the book of Malachi. Malachi was the last prophet before John the Baptist, approximately 400 years before Christ. Who was Malachi writing for? If there was no other prophet for the next 400 years, God must not have been moving and working during those years.
Both Malachi 3, which speaks of tithes and offerings, and Chapter 4, Malachi speaks of the Lord's 1st and 2nd coming. Therefore, I assume tithes are applicable during these time periods.