Power limits borders are set in BIOS of GPU and measured as 100±X, where X is separate for positive and negative offset. (more like 6//15 = from -6 to +15%). Technically you can tweak these numbers with MPT and registry edits (as well as base power limit, which is 100% and measured in watts, which is complemented with TDC, and it is measured with amperes)...
But by default, 70 or 80% power limits is impossible to set. You can only set offset from 100% you are ABLE to set.
-2% means power limit will be 98%. To get to 70 or 80 you need to set -30% or -20% offset respectively. But for most consumer GPUs negative offset hardly goes below -6% or -10%
Resulting power limits in watts will be X (nominal power limit = 100%) * (100+X)% where X is what you set in driver settings. For example nominal power limit is 100W. You set offset to -7%. meaning resulting power limit in watts will be 100*(100+(-7))%=93W
