Before you start rasping off wood, which won't grow back by itself, you need to visit a pattern board. If you are confident that your mount is consistent, this is what you do:
Measure out 16 yards, exactly 48 feet from your eyeball to the pattern board.
Mount and fire roughly half a dozen shots at the same mark. Put a mark on the target to fire at, but don't dwell on your aim, just mount and fire as if it were a live bird.
If your gun mount is consistent there should be a definite centre to the pattern. If your pattern is not centred on the "bird", measure how high/low and left/right it is.
That measurement in inches, will translate into sixteenths on the gunstock. If your pattern is 4 inches high, you need to lower the comb by four sixteenths, or 1/4". If the pattern is 3 inches to the left, the stock needs to be bent 3/16" to the right ( cast off ).
The traditional way an English smith would alter the stock would be to heat in oil, bend it, then clamp it in place until it sets in it's new position. I'm not sure how well that technique would work with a through-bolt.