This formula can be found from a patent related to parallel combs structure. The formula places the first echoes coming out of parallel combs to uniformly distributed sequence. If T_ ,...,T_n are the delay lines in increasing order, the formula can be derived by setting T_(k-1)/T_k = Constant and T_n/(2*T_1) = Constant, where 2*T_1 is the echo coming just after the echo T_n.

I figured this out myself as it is not told in the patent. The formula is not the best which one can come up. I use a search method to find echo sequences which are uniform enough for long enough time. The formula is uniform for a short time only.

The formula doesn't work good for series allpass and FDN structures, for which a similar formula can be derived with the same idea. The search method works for these structures as well.