If you are going for the Gamblers Crown then you should be seeing these numbers
Percentages: On R41+ gear using Grade I Materia
2-Slot will be 40-42%
3 Slot will be 35-38%
4 Slot will be 25-30%
5 Slot will be 18-23%

This is proven on any gear over 41 and not HQ.
Gear/weather/moon/direction/type all have 0% effect on the meld
What does effect it"
strength of the materia (is it +1 str or +5)
gap in tier(tier 1 materia on a tier 4 item)
HQ item or not.