How can I determine from the tax form 1040 what number I’m supposed to look at to calculate maaser?
The best way to handle this is to put aside 10% of each check that one receives as it is received. The money that is withheld for tax purposes is not counted for ma'aser. As a matter of fact, any money that one receives, even if it is tax free is subject to ma'aaser. That is why the form 1040 is not really valid to use in calculating ma'aser.
Thursday, 11 May 2006 Rabbi Dovid Bendory
There is no such thing as "maaser-free income" — we owe maaser on ordinary income, interest, and gifts — even on tax-free interest. Even a poor person who survives on donated tzedaka — in other words, all of whose income comes from money that itself has been given as maaser — is required to give maaser on the tzedaka he receives.
Thus you can see that even money that does not show as income is still subject to ma'aser. I treat any tax refunds as subject to ma'aser when I receive it even though it does not appear on the following year's form 1040.