There are guaranteed slots on gear that have a 100% success rate. You can get around +80CP on your gear without worrying about RNG.
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Plus, the lower grade materia you slot, the easier it is to overmeld. I keep around 30/40-ish Grade 3 CP materia just to finish up some gear that only accepts like 1 CP anyway. If you have the time, desire, and money to overmeld with full 8s and 7s, you're probably a hardcore crafter.