I have probably gone to massive overkill so I might let others pipe up with any advice they have. The old advice used to be a cubic meter of concrete to ensure stability and lack of vibration, so I went with that for the two bigger scopes (A 10"F4 Newt and a friends longish refractor) though the second hole got wet and started to have the sides come in so it was more like 1.2M of concrete as paying for some extra was easier than trying to form up a box around a meter and then backfill and compact the rest.
I chose not to then use Sonotube extension to form the concrete up to the pier plate, I prefer the flexibility of being able to unbolt the pier and put on a different height or design if I want to so I stayed with a flat top and used Chemset to fix bolts in place.
I plan two more piers for smaller scopes (Shorty refractors both) so I will probably cut it to a smaller size like 1000X1000 by 600 deep for those, to get stability that would probably be good enough for the big scopes anyway, but certainly good for a 380mmFL refractor.
I will say that the big lump of concrete actually feels noticeably "Dead" to walk on, and you can jump up and down on it while guiding without seeing any noticeable effect.
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