CR,
Everything I have read on building reduced power 12 ga loads indicates that you need to keep pressures above 7000 psi to avoid inconsistent velocities and patterns. You don't say which wad you are using, but Alliant data lists recommended 7/8 oz loads with Win AA shells and Red Dot as follows:
Wad Grains Approx. psi.
Win. WAA12SL 16.5* 7300
Win. WAA12L (gray) 16.5 7900
Fed. 12SO 16.0* 8000
Rem. TGT 12 16.5* 7300
* = add one 20 ga 0.135 in thick card wad to inside bottom of shot cup.
Alliant lists MEC bushing capacities with Red Dot as follows:
Bushing# gr of Red Dot
21 = 12.1 gr
23 = 13.1 gr
24 = 13.7 gr
26 = 14.9 gr
My 12 ga recipe, using the Claybuster #CB 0178-12 WAA12L Replacement wad, 16.4 gr of Red Dot (bushing # 28), 7/8 oz shot, Win 209 primers, and Win AA shells, kicks about like a 3/4 oz 20 ga field load