Always meld a lower tier materia first.

For example, a PIE III followed by a PIE III would give you around 30% chance. If you go III then IV chances drop to 25% and if you do IV then IV chances can be 22%.

The lesser materia first is the rule. HQ items have better chances of success in my experience. I am not talking about percent only. I just double melded more HQ items then NQ, that's all.