isn,t it simply L/D?
Assume still air, ISA, M0.78 and FL360. Aircraft a) = 60000kgf; b) = 50000kgf. The L/D for a) is 16.98 and for b) is 15.84 so that all things being equal aircraft a) has the advantage of cleaving through the air more efficiently at ToD and therefore must commence its descent earlier. QED?