Could you make a simple statement describing the law of tithing?

Gramps,
I have a tithing issue for you, several of us at dinner the other night were discussing the law of tithing, and as with any discussion it seems there were more opinions and idea’s regarding the law of tithing than facts. I’ve since re-read D&C 119 and any other books that I can find and I feel like I have a good grip on the matter, but am having difficulty explaining it to others. Any help you could be in very simply stating the law of tithing would be most appreciated.
James, Lake Point, Utah

Dear James,
Off the top of my head— Tithing is 10% of one’s increase. Different circumstances obtain for wage earners that for business men.
Wage earners– You take your monthly pay check, fixate on the Gross Income figure, usually on the left side of the check. Move the decimal place on that figure one point to the left, make out a check for that amount, and send it to the bishop. Nothing to it.
Business men– Itemize all personal income from the business, whether received as cash or paid by the business. For instance, some businesses are so arranged that personal taxes are withheld, personal insurance payments are made, and other personal expenses are disbursed from company funds. This of course lowers the profits and therefore the taxes of the company, but in fact the company is disbursing the individual’s income–part of his increase. That is not an appropriate dodge, however, to paying tithing. One’s increase from a business includes all benefits to the individual from the company. An appropriate tithing would be 10% of that amount.
Now. when paying bills at the end of the month, the tithing check, our debt to the Lord, should be the first check written. Thus there will always be money to pay the tithing. So it does not even require faith to pay one’s tithing. Where faith comes is in trying to live for the rest of the month on what you have left.
Gramps