I indeed have to strongly disagree with this 4% per lv.6 ingredient, as no large scale experiment ever was made public. This kind of linear-law reasoning is exactly what made people believe 11 element resistance were needed to counter 1 element attack, which is, as proven by a few users, completely nonsensical.
While I cannot assume anything regarding your studies, age or anything along those lines, I still dream of a world in which everyone understands that nothing is simple... Strength to Attack is linear (or at least well described as linear), so is Intelligence to Magic Attack, but... I am afraid that is about it. Linearity in a game that is all about optimizing a character has no value, as shown by enchantments and soulcrafts : the higher you go, the higher the bonuses for each rank you pass.
In this case, you could very well assume that chances to craft the hidden item are as follow : 0.5% per lv.6 material until 20 (amouting to 10%), then 10% more for 21 (20%), then 20% more for each other ingredient up to 25 (100%). Or some even harsher rule, such as an exponential function, anything ! You never know with developers.