I have to stop thinking .......
One last question and I'll stop bugging you both I promise 
Questions and discussion are the lifeblood of Tech Log ..
I understand down slope is the most limiting
For obstacle considerations, why should this be so ?
is it valid to add the slope to the published gradient
Why would you want to do this ?
Once the aircraft is airborne, it has no interest in runway slope (except for the case where runway slope equals or exceeds climb capability .. which can be of interest OEI). While we need to factor in slope re screen RL what the extended centreline slope might be doing further out is of little interest other than to the extent of providing critical lumpy bits...
Takeoff obstacle clearance is based on screen position, climb with respect to the airmass, and whatever the ground clearance may happen to be.
I suspect that I may have missed your point along the way .. in which case your response should get us back onto the discussion highway.