mom3, This is a very interesting topic. The way it has been handled and continues to be handled is a great example of how to confuse. As pointed out in the (incomplete) wiki article, to "be tithed" was not a definite amount. There was a continuous tug-of-war in the first 50 years of the Church between whether a person was tithed on their net worth or was tithed of their surplus, so even the basis was in question. When it was surplus, it was always 100% of surplus, when it was net worth, it was a varying percentage, though 10% became the norm. I believe these were both trying to arrive at the same basic idea: that some percentage of your net worth was surplus. If you look at D&C 119, here's what is in there:
1 Verily, thus saith the Lord, I require all their surplus property to be put into the hands of the bishop of my church in Zion,
2 For the building of mine house, and for the laying of the foundation of Zion and for the priesthood, and for the debts of the Presidency of my Church.
3 And this shall be the beginning of the tithing of my people.
All surplus. All. What was surplus? That was up to interpretation, but the instruction was "all surplus" (v1).
According to v3, this is only the first step. There's another to follow.
4 And after that, those who have thus been tithed shall pay one-tenth of all their interest annually; and this shall be a standing law unto them forever, for my holy priesthood, saith the Lord.
Notice here that those who gave all their surplus in v1 are now said to have "thus been tithed". In other words, tithing of surplus in this revelation means to give all surplus.
After that first step, then 10% on interest annually is a separate law. This is really where we get the "Law of Tithing". Notice that this portion of the revelation, the law, is not 10% of surplus.
Then the revelation addresses the issue of those who will come to zion later:
5 Verily I say unto you, it shall come to pass that all those who gather unto the land of Zion shall be tithed of their surplus properties, and shall observe this law, or they shall not be found worthy to abide among you.
"Be tithed of their surplus" means to give all their surplus (see v1 and v4). "Observe this law" refers to then paying 10% of their interest annually.
I know that there are people who are trying to promote the idea that 10% of surplus was the original law, but that's simply not accurate. It relies on the misapplication of the term in v5 "shall be tithed of their surplus", but in this context, it clearly meant something much different.
There was a thread a while back that introduced the idea of 10% of surplus along with my more detailed post of the history of the calculation.
The bottom line for me is that if anyone wants to pay 10% of their surplus, that will be more than I am paying, so good for them. Just understand that there is no historical or theological precedence for it; the Church has never seen it that way or taught it that way. The Church has been all over the map on how to calculate tithing, but that is a stop they've never made.