From the diode equation,
I = Is*(e^(qV/kT)-1) where Is is the reverse saturation (leakage) current
I/Is = e^(qV/kT)-1
I/Is ~ e^(qV/kT) for I>>Is
qV/kT = ln(I/Is)
V = (kT/q)*ln(I/Is) = (kT/q)*ln(I) - (kT/q)*ln(Is)
dynamic resistance = dV/dI
dV/dI = (kT/q)/I ; k is Boltzmann's constant, q is the charge on an electron in Coulombs, T is temperature in degrees Kelvin
kT/q = .026 at T=300 Kelvin
dV/dI=.026/I at T=300 Kelvin
The number I remember is (26 ohms)/(current in milliamps)