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Doug,
From my understanding, you want to pour a footing in the ground, and then continue up with a short column, at the top of which there will be 3 bolts for your base plate. My suggestion would be to do this in 2 stages. Excavate and pour the footing first, and embed 3 x 12mm starter bars 600mm long into the footing, so that they are exposed by 300mm. (ie 300mm in, and 300mm out of the concrete) Cut a piece of your formatube approximately 75mm long, and set that on the top of your footing, and fill that at the same time. The next day, cut away the 75mm collar of formatube. You will then be left with a stub column 75 mm high that will act as a starter for the next stage, and will prevent concrete from spilling out. Slip the formatube over the stub, and ensure that it is plumb by using a spirit level on 2 faces, at 90 degrees to each other. Brace the tube well. Fill with concrete, and fit off your bolts at the top with the template of your particle board. However, make quite sure that the particle board is HMR board (highly moisture resistent, it shoud have a greenish tinge on the cut edges), if it isn't the template will disintegrate when it gets wet. Use a wettish concrete to make sure it compacts well, but don't make it too sloppy. When you have finished the concreting, ensure you check the tube again for plumb. You will need 170kg of raw materials per metre length of 300 diameter formatube, plus of course what you use in the ground.
Happy Concreting,
Mike
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