I got advised 75mm for the waney edge in my front garden but 100mm for the closeboard in the rear garden.
That said I wish I had gone for 100 all round, much more sturdy and well worth the price difference
I'd agree with Thermo, he's rarely wrong!! go 2' foot down, the extra 6" will make a world of difference in strength. If you need 6 foot and gravel board then go with 9' posts and cut them down to 8' 6" if you need.
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