I quite agree - but all I am after really is some order of magnitude
figure. Air drag is significant, but might be called a 'second order'
effect. To put it another way - I know this is going to vary from
vehicle to vehicle as you quite rightly say - but what effect does air
resistance have on the size of payload? 1%? 10%? My current program
ignores air resistance altogether, and I am just curious as to the
size of error this introduces.

There isn't any simple one-size-fits-all formula for the Drag Coefficient.
It depends on the vehicle's diameter (frontal area) length, nose shape, tail
shape, whether the body has straight or curved sides, whether it's stepped
in between stages, whether the rocket motor(s) are burning or not, and the
effects of transonic and supersonic flow.

A good place to start would be Ashley and Landahl's "Aerodynamics of Wings
and Bodies",

--
Pete Stickney
Any plan where you lose you hat is a bad plan