[CoolhandJo (Paul)]

Just a side thought/note. If you elect to lay a slab down as the base then you dont need a pier footing. You can simply lay concrete blocks on the slab (tied in with dyna bolts or cast in at time of pouring slab). Then you simply fill in the blocks with 15mpa block fill and can even cast the pier plate bolts in! This will help any differntial movement that subterranean footings can suffer with. It can also help with getting the correct height of your pier as you can elect to lay as many or few blocks as you require.

I did this as my pier was palced on an existing slab.

Note/Disclaimer: You should check the coeficient of linear expansion (temperature dependant) to ensure that the slab you lay (primarily surface dimension) has sufficent mass to a) support the above ground footing, and b) be of the same material to exapnd and contract in union.