It is the 'cup' that you want off the wad - nothing else- everything else is garbage - toss it - The cup off the bottom is the gas check. It is put cup-down to seal the powder, and one is put facing cup-up to centre the ball. All trap wads I've seen have a concave gas check seal on the bottom of the wad. I am not sure what you are talking about - flat? flat what?. Put one cup down on the powder, then the right size cushion wad for crimp height overall, then another gas check cup(OFF A WAD) with the concave facing up, to centre the ball in the bore.
; The nominal size for 12 bore is .729". Mnay today are overbored or back-bored to larger sizes, even to .740". One must slug the bore to find out what it is if you want to use the largest ball possible. The weigth goes up quickly.
; Your shotgun doesn't know the difference between a 525gr. slug and a 537gr. ball. Then, there's the weigth of the wads, length of bearing surfaces causing friction, etc. It is best to start low, of course, as you have, but you can probably work up to 1,450fps easily. THAT's a powerful load.
; It may only be 2,508FPE, but on Taylor's KO chart, it's 79.5. A .300 Ultra Mag develops a whole 27.7KO roughly 1/3rd of the real power. 1450fps is 100fps higher than the moderate, more extensively used African load for the 12 bore BP cape guns and rifles produces.