Bear in mind that the lock bolts are going to snug it tighter against the barrel flats too, but the finish and stains will also swell the wood. Inlet it until it's really really close all the way around with hand pressure. You can always come back to it and dig it in deeper. If you've already removed the wood in front of, or behind the lock, (so that the nose or the tail of the lock extend beyond the available wood) then you'll have little alternative but to file the lock bolster until you get a good solid contact front to rear.
Though experienced builders that do everything perfectly every time don't need it, lots of people use that epoxy putty or a sawdust-glue paste to build places up in they've over-inletted. It's not correct, but if it doesn't show it does work.
The most important thing is that you get a good solid contact between the lock bolster and the side flat, and that no powder can trickle in to the lock recess. If powder can get in there, then fire from the pan can. If powder AND fire get together, they can find a way to exit at a weak point, probably where the wood is thinnest, like between the back of the main spring, and in to the ramrod channel, but you never know where things will fail when you are doing destructive testing.
It's very difficult to know if your lock is 100% flush / flat or 90 degrees to the lock bolster. One way to tell is with your exacto knife, If there is no gap at the top edge, and no gap at the bottom of the pan then you're probably good. But, even a sharp knife blade has SOME thickness to it, which is certainly thicker than a gas molecule.
The even more accurate way would be to flash some powder in the pan (before you finish everything). If you can do it pre-vent hole drilling, and with the frizzen cover closed that will help build the pressure a little bit to help seek out any weak points in the contact. Don't worry too much about the pressure. If it's too much it will just flip the frizzen open. Remove the lock and look at the barrel flat. If there is a good solid demarcation line with the top edge of the lock, and bottom of the pan, and no powder fouling seems to have gone below the contact line, then you're good.