This is from a newbie, but I'm contemplating going the route of 24"-30" down, then dry quikrete into the hole, covered with dirt near the top.
Reason being, I just removed a bunch of these posts that appeared to have been set just like this, with a thin footer, about foot and a half height of concrete around the post bottoms.
From what the neighbors tell me, this fence was roughly 10 years old, built under the reign of the previous owner. There doesn't appear to be much heaving and those posts were REALLY in there solidly. No decay. PT 4 x 4 wood. I had to dig down around/under the footings in order to get them just to budge, they were in so well. Then had to hit the footing with a sledge to get it to break to retrieve the post out. Heavy sucker. If I can duplicate that, I'm sold.
It's possible they may have premixed the concrete, but that's anyone's guess. I like the dry powder route. Less muss or fuss. Stands to reason it would set better, as well. Post won't lean while it's hardening.
PS- Not that this may matter, but for soil and weather context, I live a little over a mile from Boston Harbor on the South Shore.
I have used with excellent results the Quikcrete that you just mix in the hole - pour in the mix, add water, mix it up with a stick. Easy to do and creates a solid post. I had to remove one I had done this way to install a driveway gate - it was a bear to get out...