Derek,
your man's definitely right for me on the onions but having switched to my new gas stove i've realised the amount of reduction "evaporation" at the frying stage has to be taken into account. i'd settled on 70% ish bulk veg in the finished base volume (of which most is onion) c/w to a norm of around 50% in the various base recipes. i'm now thinking the norm is right as the stove seems to reduce the base by 50% (400 to 200ml) c/w with 30% (300 to 200ml) in the past.