I concur on what the Doc says and the load data he shared.
The SP10 wad is fairly forgiving on pressures with the slower mag powders like Herco, Blue Dot, Steel, 3N38, Lil-Gun, etc. You can reduce by 10-20%, bump to a true 2oz load and work back up slowly. Should be capable of the 1,250-1,300 FPS range with a 2oz payload. What might matter a bit more than the powder, wad, and payload, in this scenario is the hull you are using. Paper base wad Feds will drop your pressures a little as well, but all of the poly base wads are going to run 200-800 PSI higher on average, so keep that in mind.
What shotgun are you testing these loads with?