I have searched high and low for a straight, accurate answer to my question, so please do not flame me if it already exists.

How can materia bonuses be measured before the melding attempts begin? I have been currently just using trial-and-error techniques, but as I will be using higher-grade materia soon, I would like to know.

As an example, I have an ilevel 16 glove with +18 gathering and no other stats. I attempted to put in a Gatherer's Guerdon I (+3 perception); however, it said that the item was too low to maximize the materia's potential, and it only gave me +2. That would lead me to believe that any first-tier materia would give me 2/3 of the power, but that's not the case! I put a Gatherer's Guile I (+3 gathering) into the second slot and got 100% of the benefit.

There doesn't appear to be a rhyme or reason... Does anyone have *reliable* information on this topic?