FWIW, when I do something like this I:

Clamp directly to the table. Use parallels or 123 blocks to keep it
slightly raised. Limit the downstroke with the stop and put some
scrap
underneath just in case.

Cut out as much material as possible before I get to the boring head
part. Then start the few thou at a time boring part. :-)

Use a telescoping gage to check progress.

Try never to change the setup until it's done.

Also, when I start to get stressed things start to go WRONG. So I try
to back off,
do something else, ...



It sounds to me like your main problem is that you are not comfortable
with
telescoping gages. They are a little fiddley so I would suggest that
you play
around with them a bit until you get the hang.

Not using a vise will make the setup more reliable and give you more
clearance to measure.

DOC