(I worry I'll regret getting involved in a Pokemon vs Fairy Tail Versus Thread Match at all, but....)
Indeed, AFAIK, we don't have other values for how much the move effects modify stats, so the in-game values are the main actual numbers we have.
There's also that non-Pokemon don't have a known limit to how high or low their stats can be modified, unlike Pokemon, but anyway.
Whee, stat stages.
bulbapedia.bulbagarden.net
For Attack, Defense, Special, Sp. Attack, Sp. Defense, and Speed
Stage multipliers
| Stage | -6 | -5 | -4 | -3 | -2 | -1 | 0 | +1 | +2 | +3 | +4 | +5 | +6 |
|---|
| Gen I-II | 25/100 | 28/100 | 33/100 | 40/100 | 50/100 | 66/100 | 100/100 | 150/100 | 200/100 | 250/100 | 300/100 | 350/100 | 400/100 |
|---|
| Gen III+ | 2/8 | 2/7 | 2/6 | 2/5 | 2/4 | 2/3 | 2/2 | 3/2 | 4/2 | 5/2 | 6/2 | 7/2 | 8/2 |
|---|
Thanks, Bulbapedia. So, unlike with increases to above stage 0, which are just each +50% of base value, decreases to below 0 have diminishing returns on the decreases the lower a stage one goes to; Going from 0 to -2 is 100% to 50%, -2 to to 4 is 50$ to 33%, & -4 to -6 is 33% to 25%.
Of course, even without knowing how it could go, further along, if at all, being at one quarter speed (or lower.) could be quite problematic.