Based on Carl's advice, I've decided to employ the origination and extinction rate estimates outlined by Michael Foote (2000). See below:
Nbt= Total taxa that cross both boundaries
Nb = Total number of taxa crossing the bottom boundary (NbL [only bottom bndry crossed] + Nbt)
Nt = total number of taxa crossing the top boundary (Nft [only top boundary crossed] + Nbt) leaving bin but not entering (i.e. born in bin)
dt = the temporal duration of the bin in millions of years (or whatever unit you want your
These are per bin rates, so the average can be used if we are assuming time homogenous birth/death rates.