Comparison between cards can be made simpler by searching for "relative performance" of the higher card. Divide the difference in % points between the cards in question by the % of the inferior card to calculate the % difference between them.
I found a graph showing performance in multiple games relative to a 2080ti @ 1080p It showed that a 1070ti = 63% of the 2080ti, and the 1070 = 56%.
So 7%/56% = 0.125 = 12.5% difference between the two cards in question
Funnily enough, £20 is exactly 12.5% of £160 so the higher price is exactly proportional to the performance increase.
Note: this is normally old data, so things will have changed with reference to driver optimisations for newer cards, but it's a good way of estimating things. You can do the same for 1440p or 4k by amending your search.