69% vs 100% is 31 percentage points, not 31%, that's not how it works.
If the 7800 XT is GPU A with the 9070 XT being GPU B then GPU A needs to gain the performance relative to its self to reach GPU B, there is an easy way to calculate that, you divide GPU B by GPU A, so 100 / 69 = 1.449, the number after the decimal point is the percentage GPU A needs to gain to reach GPU B, so 44.9%.
To proof it you can simply add the 44.9% to GPU A, 69+ 44.9% = 99.981 rounded up that its 100%.
So the 9070 XT is 45% faster at 1440P