Not sure if this is the 'right' way to do it or not, but it works;
Taper the wood shaft.
Insert firmly into the broadhead. Suggest using a broadhead wrench & much caution.
Mark the shaft at the back of the broadhead.
Remove the shaft and lay it alongside the broadhead, lining up the mark with the back of the ferrule.
Now, mark the shaft taper at the back of the bleeder slot. Give yourself just a wee bit more when you mark it.
Remove the excess taper with a fine saw or roll a sharp knife across it on a flat surface and break off.
Do a dry run to make sure the bleeders will clear & it spins true. Again, much caution.
If everything is good to go, glue it up.
After you do a few, you will probably be able to just eye-ball the amount to cut off.