Technically Christians are supposed to give as the Spirit tells them to give, there’s no rule that says we to “tithe.” I daresay it’s harder for most Christians to actually be led by the Spirit with their giving, and it’s easier to give a flat 10%. The Spirit will challenge you, surprise you, ask you for offerings that hurt your flesh. 10% of your gross with every paycheck is a constant and not as difficult.
But since we’re talking about the tithe, the book makes it clear it’s the gross.
6 “A son honors his father, and a servant his master. If I am a father, where is the honor due me? If I am a master, where is the respect due me?” says the LORD Almighty. "It is you, O priests, who show contempt for my name.
"But you ask, ‘How have we shown contempt for your name?’
7 "You place defiled food on my altar.
"But you ask, ‘How have we defiled you?’
“By saying that the LORD’s table is contemptible. 8 When you bring blind animals for sacrifice, is that not wrong? When you sacrifice crippled or diseased animals, is that not wrong? Try offering them to your governor! Would he be pleased with you? Would he accept you?” says the LORD Almighty.(Malachi 1:6-8)
In other words, you’ll pay the government whatever they TELL you to pay them, and you’ll do it by April 15th, and chances are you’re overall giving a good 20 or 30% of your income. But now when it comes to giving to God, you want to cut a deal and give Him what you want Him to have, not what He’s asked for. Your gross income is what you report when you go to get a car loan, buy a house, do your taxes, etc., and yet people get “confused” when it comes to giving God His share. Which is funny to me because Uncle Sam wastes your money. God promises to give it back pressed down, shaken together and running over and bless your descendants to the 4th generation, all for the low low price of 10%. Pretty good deal if you ask me.
He wants 10%, all of it, thus why He said “bring the whole tithe.” Not half, not 10% of whatever you had left over after you reverenced the government, your bank, your 401k, etc.
Some people just flat don’t know this, and so if they haven’t been taught I don’t think God holds it against them. But if you are going to commit to tithing regularly, hell or high water, it’s 10% of the gross.