I have sucessfully reamed several chokes using the Brownells's adjustable set by basically the method Lenard described. As he said use light cuts only & "never" turn any reamer backwards in a hole, adjustable or otherwise. The set I bought came with instructions writen by Ralph Walker. It is noted though, those directions did not differentiate as to type of choke. So again I "Warn" "IF" you have no paralell section to start with you are purely operating on "Luck" to maintain alignment. I know Lefever, fox & i believe several other US makers used the "Taper" choke systemn. I would strongly recommend against using an un guided reamer to open one of these. When you make that first cut, you have only one point of contact & no way to insure alignment. Once started crooked it will then continue to follow the hole you have started.
