The elevating gear is a quadrant gear, and it is centered on the trunnion centre. This is how I set it up on the model Armstrong RML 80 pounder. There are probably other better ways of achieving the result, but this is how I did it .
This is the second 80pd Armstrong RML model which I am finishing, this time for myself. It should be straight forward, having done it all before, but it is like a new adventure. Ah the joys of memory loss. Everything is new.