If this is a climber, start with the platform angled up slightly. The more the difference between the diameters at the bottom and where you want to be, the more angle to put on the platform. If it is a fixed stand, just tighten the chain/strap. If it is a fixed, chain-on stand, this sounds dangerous, because if the platform is angled down, that means the gripping teeth are not up against the tree very well.

As for a formula, put a tape around the tree at the height you want to be and divide that by 3.14 to get the diameter. Most stands aren't made for trees less than an 8" diameter. The angle of the gripping teeth won't allow enough contact for a solid hold.