Minor save/power tiers tweak

[lukep]

With the new saves system, tiers are quite important, but I noticed something a bit odd with them: they would tick over to the next higher tier later than I was expecting. Specifically, they changed at 21, 41, 61, and 81, while I was expecting 20, 40, 60, and 80 to be enough to reach the next tier. Changing this would make these edge cases clearer to me, and be more easily understandable.