I've been using the rocket equation, 9.2 km/s to LEO and the 4%, 2% numbers given by Elon in my attempt to calculate the fuel used for reusability, staging at Mach 6 for the reusable vehicle.

My question is, "How much of the gravity/aero drag allowance should I add to the staging velocity to have a more realistic impulse from the first stage?"

Mach 6 is about 2 km/sec, but if I allocate all of the drag to the first stage then the rocket equation would calculate a staging velocity of 4 km/s, 3/4 of the drag to stage 1 would give a staging velocity of 3.5 km/s and so on. What is the best guess allocation of drag between the first and second stages?