Quote:

"The simplest statement we know of is the statement of the Lord himself, namely, that the members of the Church should pay 'one-tenth of all their interest annually,' which is understood to mean income. No one is justified in making any other statement than this" (First Presidency letter, Mar. 19, 1970).

In other words, the way you define your income, and consequently your tithing, is a matter between you and the Lord. Prayerfully seek the Lord's guidance on issues like taxes, gifts, scholarships, and other matters to determine what qualifies as a full tithe.

Here we have the official position of the church. The way you define your income is a matter between you and the Lord. Therefore, I am free to decide whether this means gross, net, surplus, or every tenth green jellybean I get, as long as I believe the Lord approves.
Church culture, however, says it must be 1/10th of the gross income. But the LDS Church does not get a copy of the W2, and trust each individual to work out their tithing with God.

What's more, the church has allowed multiple LDS authors, some of them Apostles, to write books which contradict each other. They allow for freedom of the press, and dissenting opinions, so long as it does not extend to slandering the leaders, or calling for the overthrow of the church.

- Dragon