Original Performance is 1.0.
An increase of 70% means: 0.70 more in addition to the original performance of 1.0, so you're at 1.70.
Correct^
In that, you add them BOTH, to get the SUM = (x 1.7). We are talking only about the 70%. (Not the sum of both.)
So 70% = x 0.70
Not to mention, he was trying to multiply percentiles. 50% + 50% is different from 50% x 50%. This error was skewing his math and throwing off his calculations. I just wanted to bring this to his attentions.
If you EVER have any doubt, just type the equation in @Wolfram|Alpha. You can just type in words if you want and ask W|A.