The tithe is based on 10% of the first fruits of income or increase that comes into a person’s life.  Here is the promise of the tithe.

Malachi 3:6 For I am the Lord, I change not; therefore ye sons of Jacob are not consumed.

7 Even from the days of your fathers ye are gone away from mine ordinances, and have not kept them. Return unto me, and I will return unto you, saith the Lord of hosts. But ye said, Wherein shall we return?

8 Will a man rob God? Yet ye have robbed me. But ye say, Wherein have we robbed thee? In tithes and offerings.

9 Ye are cursed with a curse: for ye have robbed me, even this whole nation.

10 Bring ye all the tithes into the storehouse, that there may be meat in mine house, and prove me now herewith, saith the Lord of hosts, if I will not open you the windows of heaven, and pour you out a blessing, that there shall not be room enough to receive it.

11 And I will rebuke the devourer for your sakes, and he shall not destroy the fruits of your ground; neither shall your vine cast her fruit before the time in the field, saith the Lord of hosts.

12 And all nations shall call you blessed: for ye shall be a delightsome land, saith the Lord of hosts.

Interestingly, there is a 10% rule regarding energy.  You can learn more here.  The video will stall out (at least it does at the time of this writing) if you are not registered, but, you can scroll down and read the accompanying article.