Brian, back to the original question. You wanted to have a rough idea of the "negative" lift of the tailplane. It shouldn't be that hard to work out, since basically the main wing holds up what gravity and the tailplane are pushing down. And how much the tailplane is pushing down depends on the distance between the C of G and the main wing.
So if you know the mass of a given airplane and the C of G, and you know the exact location of the MAC (mean aerodynamic chord, more or less the center of lift) of the main wing and tailplane, it's a fairly simple calculation, which closely resembles a normal M&B calculation.
This will give you a rough number. To refine it, you need to factor in things like:
- Center of lift of a wing changing with its angle of attack
- Fuselage-generated lift
- Center of lift changing with flaps & slats extension
and so forth.
But for a rough number, if you can cough up the mass and C of G for a given airplane, and the MAC for main and tailwings, I can show you how the calculation is done.