Subtracting clamp curves from pull-up/down curves

Hello IBIS gurus,

When creating a model for a three-state device, it is necessary to subtract the
clamp curves from the pull-up/down curves to avoid double counting, as the EDA
tool adds the clamp-curves back in to the pull-up/down curves to obtain the
devices output charactersitics in the enabled state. Right so far?

The question is this, do you only subtract the corresponding clamp curve from
the related pull-up/down curve, or do you subtract BOTH clamp curves from each
pull-up/down curve?

For example, if I am creating the pull-down curve, do I subtract only the
ground-clamp curve, or do I subtract both the power-clamp and ground-clamp
curves? Does the EDA tool add only the ground-clamp data back in, or does it add
both curves back in?

-- 
Regards,
Stephen M. Nolan