I've occasionally thought about building a fancy 4 sided jig with threaded rod that acted as a collar, but I'd have to be doing hundreds of these in a row to justify the jig.

Clamp a longish (width of post plus 20") piece of straight wood, parallel to your line and offset down by the number you measured.

Repeat on the other side. If the post is 6x6 and you've got a normal saw, cut two more times, then pull out the reciprocating saw.

You've probably heard the term "practice makes perfect" but like many people, you may never have applied it. Burn through a few posts and you'll be a lot better at cutting them off with the equipment you have.

It looks no more accurate or professional than what i'd do [and have done] in a pinch with no time to spare. I'd like a more reliable higher quality result but without additional specialized equipment most preferably. Any tips?

Figure out the offset from blade to edge of your circular saw shoe. (Pick a side -- it isn't critical, though I'd opt for the longer of the two.)

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Austenite may contain carbon in any proportion up to about 2.2 It is non-magnetic, and, when preserved in the cold either by quenching or by the presence of manganese, nickel, &c., it has a very remarkable combination of great malleability with very marked hardness, though it is less hard than common carbon steel is when hardened, and probably less hard than martensite.

On cooling into region 6 or 8 austenite should normally split up into ferrite and cementite, after passing through the successive stages of martensite, troostite and sorbite, Fe 0 C= Fe 3 C +Fe(i 3).

Draw a perpendicular line on 3 sides. (Edit to reflect comments: if you prefer a level cut, use a level to draw 4 lines around the post. In practice, the difference between level and perpendicular will be negligible. A level cut could be helpful if you're putting a level board on top of the post.)

An ad from Simpson Strong Tie for various materials has a brief shot of a guy cutting a post with a circular /"skil" saw. Note: that's his arm / hand in bottom center holding the saw by brute strength and creating the "level" cut by pure eye/skill.

Martensite, Troostite and Sorbite are the successive stages through which the metal passes in changing from austenite into ferrite and cementite.

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