a wrote:

Apologies to Mr. Gabriel.

I'm installing a 4X4X16 pole for a clothesline. I'm down to about 3
feet and I hit solid rock. Not 'a' rock - just rock. I'll be using 10"
sonotube (I have 4' of it) and concrete.

Any rules of thumb for how deep I *should* be for good lateral
stability? I can obviously cut the pole shorter, but I'd like to have
10'-12' above ground to be somewhat level with the mounting point on
the house.

You need to consider how deep the frost line is and whether the land
gets soggy in the Spring rainy season.

Concrete simply poured around the 4x4 doesn't provide much support. But
if you spread it out so it has a large horizontal flange around the
pole, that will provide more bearing surface. Of course, the concrete
has to extend down around the pole in a thick enough layer so the pole
doesn't twist out of it.