I lkike to have 12-18" of solidly compacted inch minus gravel under them, depending on all the other various soils, load, and water factors.

Excellence is its own reward!

I had my camp jacked and levelled last year, including new piers. What they did was new to me but made good sense, This was a very experienced team at doing this sort of thing. First, they dug out the old piers and dug the hole down to four feet, which is the frost line in our area, Then they poured in a bag of quickrete, dry. This is to use as a leveling agent. then they made up a 16 inch square by 3 inch thick base of .6 pressure treated wood. This is far superior to the normal .4 you normally get at the home supply store. They made these bases from 2 by 8's, cut 16 inches long. Put 2 side by side, then nailed the other 2 crosswise to these. Then they put in a 6 by 6 post, again .6 pressure trated, and toenailed to the base piece. The base piece was levelled and the post plumbed. Then they toenailed the top of post to the base of the camp. A very efficient method, no mixing of concrete and waiting for it to dry. You have already chosen your method, but this might be a good idea for others.