I am inletting and glass bedding a mark X action into a McMillan stock. How much relief do I need to have behind the receiver tang to prevent any chipping or splitting with recoil, and not have a unsightly gap when finished? Would one or two layers of electrical tape be enough? Thanks.

three layers would be better. I would rather have a little gap there than run the risk of a split stock. Or... use two layers & just undercut that area a bit.

I agree with three or 4 layers of tape. But I cut and remove the tape near the top of the tang. I use release under the tape, and it stays in when the action is removed after bedding. I remove the tape after the action is out. The bedding compound is stained or dyed to match the stock when it is mixed. This leaves a void below the surface, but the upper 1/16 or so is tight to the tang. The bedding is flexible enough so it never cracks and the tang looks like it is tightly bedded, with no gap. If the bedding bulges, I know the action has moved a bit in the stock (it has happened), and I retighten the action screws. Works for me, but your milage may vary.

Roger