Use WhatToMine.com. The best available mining calculator which tells you about the current most profitable coins to mine according to what GPU you have. Moreover, it keeps you updated with the new mine-able coins popping up and you much revenue (in BTC and USD) you can earn weekly and monthly after your electricity costs have been deducted (of course, you put the cost of your electricity as well, per kWh).
Try it out.
In general terms. How assertive is Whattomine? 80% presicion?
I think the numbers on that calculator are too optimistic
They're 100% accurate and produce the results according to the
current "price and mining difficulty of the coin". However, the thing that makes the actual output/coins/earnings you receive less precise and less accurate is the variation of the price and difficulty of coin while you mine it.
Suppose that WhatToMine.com projects $10/week earnings through your each GPU for Ethereum, but till the time you actually mine it and receive it in your wallet, Ethereum price might go down, which can lessen your per week earnings. This is the only thing which can make your overall calculation through WhatToMine.com inaccurate, otherwise, it's always accurate and according to the current rate.