The % change not by the level but the amount of what it give

Try this IV STR with 16 str and then try a III with STR 14 or 15 and you would see that you gain 1-2 more % so using lower version does not help it also have to do with how much it give. Gear would never help your % and it wont ever maybe one day but for now the only way is to keep trying to add stuff over and over and over.