so that's my high dollar pin drilling alignment rig. I measure to the tab while the barrel/tg is out...making sure I use a felt tip marker on the barrel either side of the square. I install the barrel, put the flat in between the marks....put a touch black on the sharp tip, push into the wood, flip to the other side, repeat.
I use my smallest gimlet to start the hole, switch to drill and drill to the tab. Take the barrel/tg out finish drill thru the tab, reinstall and use the tab hole as a guide. It almost always comes exactly on the black mark on the other side.
Doesn't matter if the wood is flat or curved.
If I can't drill a straight hole the 1/4 or so from the black mark to the tab, I probably shouldn't be building firearms.