I note that the calculations assume that leaf tuples and internal tuples have similar sizes. We calculate the average leaf tuple size, and use that to calculate the fan-out of internal pages. On some GiST opclasses, the values stored on internal pages might be quite different from the leaf tuples. I don't think we need to worry about that in practice, these calculations are not very accurate anyway, but perhaps a comment would be in order.
Probably we could collect per-level statistics of average tuple size?
