whattomine seems to be using at least the wrong network hashrate for its calculation.
Luckily there are better solutions.
I like the calculator @ altpool.pro but It works only for equihash and ethash
There is also the calculator @ crypto-coinz.net which includes more coins and algos, but i don’t understand how they calculate exactly. It seems to calculate the last 24 Hours.
Maybe you have better calculators you can share too.