Tang holes are drilled "to a point", the Greener FP action is different than most A&D actions in that the female portion of the hand pin is a bind hole, not threaded through the trigger guard boss. A slave pin was turned to mark the location of the blind hole whole while a point was turned on a pin which fit tightly in the top portion of the tang hole.
Slave pin

Blind hole marked

Lining up the points

The full set up

New slave screw for final inletting of the trigger plate

The rear tang screw will be drilled in the same fashion, more pics to follow.