Some relevant bits to this discussion on ABC mechanics on Merlin II/III from this manual:
Further complication arises since reference to both Fixed Datum and Variable Datum Boost control is made. I think its safe to assume in the case of the Spit II it would be the late variable datum type. In either case with ABC out you are only going to get +12lbs thanks to the Bleed holes (Mod Merlin 154)
A little more on the gate issue from the 2nd addition of the AP2095:
So the rocking past the gate (for Take off) is in effect disabling the ABC however Mod 154) still ensures that only +12 to +12.5Lbs will be the max obtained.
There is discussion in the Merlin Manual about the Boost controller requiring an initial throttle advance of +0.5Lbs to get it in operation. Maybe this goes some way to explain the 12Lbs v 12.5lbs diff ?
My head now hurts
