How do you work out performance? It's no problem when comparing Nvidia cards to other Nvidia cards, or AMD cards to AMD cards. But, benchmarks and games can vary quite a bit depending on which brand card you use.
Is there a benchmark that doesn't favour any brand of card? And, we hear time and time again that none of the benchmarks are any sort of indication of game performance.
And then what resolution or graphic settings do we use? There is always games etc that one brand card is faster at lower resolutions than the other. And some cards have better performance at some at higher resolutions.
Sorry, those are just my some of the thoughts I have when I see people talk about price/performance.
Anyway, back to your post, You did a calculation on one benchmark and claimed that the price/performance of the 1070 and 480 were the same. But one benchmark does not even begin to tell the story of the price/performance ratio. You are also working it out without knowing what the actual performance of the 480 is in said benchmark.
So you might end up been right, but, you can't make any predictions just yet.
Just want to add, that I don't think price/performance ratios work at all across different price tiers. Because the people that buy £400 cards will be different from the people buying £200 cards.