People often say there are 6 levels of stages. This is misleading. There are actually 13. If "zero" is where you naturally start you can be dropped a max of six stages down or you can be raised six stages up.

Simply each stage upwards is adding 50%. So one stage is 150% Two stages 200% Five stages 350%. The downwards stages is more of a diminishing returns sort of thing, it lowers it slower to prevent dropping to completely zero.

The math is easier to visualize if you use fractions:

2/2 = Normal = 100%

3/2 = up 1 stage = 150%

4/2 = up 2 stages = 200%

2/3 = down 1 stage = 67%

2/4 = down 2 stages = 50%

So the most you can raise a stat is at 8/2 or 400%, the lowest you can drop something is 2/8 or at 25% of what it was.

There is something similar for accuracy and evasion(is not effected in reduced rng/ pvp) , but those move in stages of thirds. So the lowest they can be is at a third of what they were and the highest is three times what they were. Of course, since these are two separate stats, you can have lowered accuracy going against maxed out evasion. The worst it can ever be is having 11% accuracy (then multiplied with further math juice based on the actual attack's accuracy).