Easy... make a "saddle" out of 1x4 or 1x6 that is basically
a three sided box that "wraps" around the post in question.
Clamp the "saddle" in place.

Use your circular saw and the saddle as the guide.

A smaller circular saw will be easier to handle.



I have a need to cut from one to three inches off 4 inch machined
round softwood fence posts that are already in the ground. There are
over 200 of the things...I want the ends factory cut square and neat,
any ideas on the easiest way to do this? Hand sawing them will be
very time consuming, but I have power available that can reach all of
them (petrol generator). I was thinking of a big circular saw? Depth
of cut is the issue with those however. Thanks for any replies