You doing this for measuring purposes only.
Bathroom sink tailpiece too long.
I suggest that you do dry run meaning assemble and hand tighten all drain pieces including tail piece first.
A pipe cutter would be a bit more suitable than a hacksaw but if you only have a hacksaw just clean up well with a file after sawing.
There is lots of play so no worries.
Cut the extension to the proper length.
New tailpieces come quite long so they can be trimmed to fit.
If it s too long cut it with a hacksaw.
Trim the tailpiece to fit.
Then cut tail piece accordingly.
If it s too short add a tailpiece extension which is a short length of pvc pipe.
Slide it onto the tailpiece and.