Ouch! :headbonk:
O.k., I find it easier to drill the slot into the main handle piece before I glue everything together. That way you may not need extra long drills, and it's easier to stay straight with the shorter travel. If I use spacers, I also predrill everything. Of course, you need to be careful that you align everything correctly and things don't shift while the epoxy sets.
Since I often don't know what the knife tang looks like, I oversize the inner tang slot slightly to accomodate twisted or curved tangs. You need a bit more epoxy that way to fill it up, but you also have a little wiggle room if the slot is not perfectly straight, which can still happen with a drill press at the price level I can afford. Depending on the size of the handle, I may use 1/4" drills or even wider - I have seen some pretty crooked tangs... If you have the knife in front of you, you can work much more precisely - Marko us going that route and may have more tips there.
Into the ferrule piece I drill a thinner, undersized slot. That slot then gets filed open to snugly fit the tang. In a perfect setup (as if this did ever happen...), you would then fill in epoxy, insert the tang and it would be straight because you filed a perfect fit into the ferrule.
I'm bad at estimating how much epoxy you need to fill a handle, and mine always flow over - keep a wet rag handy. Some epoxies shrink when they set, so You will have to apply some more later, and that can then be a different kind or one mixed with wood dust or dye - Dave is the master of this.
I hope this helps,
Stefan