Could you give me an example of a formula which would use the percentage of DEF?

my advice would be to change the way the skills are calculated at the point of action, to always include attack, defense, or whatever you want, relative to the actors, and not provide those values through damage formulas.

you'd only provide the skill's own damage value in the damage formula, and all the calculation would be done by the battle system, not the skill objects.

that would potentially evidence any imbalance in the actor's stats, because if a skill's power and the calculation formula are both correct, then the greater the difference between the attacker's ATK and the defender's DEF from the database stats, the more total damage it'll come out of that process.

set the formula to the lowest possible outcome, set the skills to 1 power, and adjust the balance through the actors'stats.

once you have your flavor and color, then vary that by varying the skill power.

sometimes, working with low stat values can yield results of 0 damage, because some calculations divide small integers by percentages, and those percentages turn out as decimal fractions, which are rounded to 0.

shifting all those calculations to one unified operation, should help you calibrate the lowest threshold.