Mr. Robot uses the actual haste % that is necessary, 12.5%. 12.5% is the amount of haste you want for that extra tick. 916 is a number that I have seen posted on multiple forums... so we quote it for clarity since so many people are used to seeing that number. I honestly have never gone and checked it myself, so I will do so now:
The math would look like:
Haste on gear * haste buff = 12.5%
So, haste on gear = 12.5%/haste buff = 1.07143. 7.143%*128.125 = 915.18. So, you would indeed need 916 Haste Rating to achieve 12.5% with raid buffs.
Hope that clears it up.