Diffusion capacitance approximation for bipolar power transistor
Good old Sedra and Smith fourth edition, pg 322 has the formula as \$C_{de} = \mathrm{T}_{f}*g_m\$ and has a note that \$\mathrm{T}_f\$ is usually in the 10 to 100 picosecond range.  It looks like your formula is equivalent.