Sniper doesn't increase the critical hit ratio, it just increases the damage dealt when hitting a critical hit and deals 50% more damage than it would otherwise.
Critical hits this gen have a 1.5x damage multiplier, which was brought down from 2x from the last generations.
These are the critical hit ratios:
The +0, +1,... you see are stages which if achieved give a higher chance of landing a crit. Here are the ways to increase the stages:
Hope I helped.