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Here is my sketch showing what I would recommend for a really steady pier that would support pretty well anything. The only reason I have suggested 300 dia formatube is because that was what Doug originally said he was using, but 200 would be adequate. I would not suggest running the formatube down to the bottom of the excavation under any circumstances, as over time the cardboard will either rot away, be eaten by termites, or otherwise dissolve, and then the column will be loose in the ground. Not what you want! If it is the intention to concrete a slab around the column, or to put a timber deck around it, make sure that you leave a 12 mm gap all around the pier, so that vibrations are not transferred to the pier.
If you have any questions, or want to have a copy of the sketch emailed to you, PM me with your contact details, and I shall send it on.
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