A general variance predictor is presented for a Cavalieri design with slices of an arbitrary thickness t >or= 0. So far, prediction formulae have been available either for measurement functions with smoothness constant q = 0, 1, ... , and t >or= 0, or for fractional q in [0, 1] with t = 0. Because the possibility of using a fractional q adds flexibility to the variance prediction, we have extended the latter for any q in [0, 1] and t >or= 0. Empirical checks with previously published human brain data suggest an improved performance of the new prediction formula with respect to the hitherto available ones.