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
I do find there is a 10-20% variance. Besides when coin stats are wrong, I think they don't take pool shares into account properly... most calculators are a bit off though so just have to take them with a pinch of salt