Short version:

If you're looking to have your clans treasury grow at X percent of income weekly, use "Percent of Gains to Treasury + Previous Reserve"

Regarding percent modes which take into account the previous payout's reserve. The percent of gains to the treasury makes the assumption that the current given treasury value includes any previous reserves, and thus subtracts the previous reserve value from the given treasury value before calculating the target percent to reserve. The previous reserve is then added to this reserve value. The formal for this is (Percent of (current treasury - previous reserve)) + previous reserve.

Without this adjustment, if the treasury value includes the previous reserve, even if your clan makes the same amount of gold each week, the pay will actually decrease. For example, say you have the reserve set to be 10% of the treasury (without adjustments) and you're making 10,000 gold a week. The following would occur (assumes the entire potion of the treasury which wasn't reserved is paid out each week):

Week Treasury Paid Reserve
1 10000 9000 1000
2 11000 8900 2100
3 12100 8790 3310
4 13310 8669 4641
5 14641 8536 6105

Whereas, with adjustment, the result is as expected with the amount paid out remaining consistent (because the income is consistent), and the amount reserved growing in a logical manner.

Week Treasury Paid Reserve
1 10000 9000 1000
2 11000 9000 2000
3 12000 9000 3000
4 13000 9000 4000
5 14000 9000 5000

However, there are time at which it may be desirable not to have this automatic adjustment, thus the option.