Industry production efficiency (or How to work out how much steel from iron ore?)

In OpenTTD - I've noticed that delivering 120 tonnes of iron ore (for example) does not convert into 120 tonnes of steel. This appears to hold for other manufacturing process in the game. I suppose this is reasonable, as there is probably inefficiencies.

I want to be able to accurately set the length of trains taking steel away so they are immediately fully loaded and ready to roll once the raw material is provided (so, 120 tonnes of iron ore rolls in, and the steel train immediately rolls out).

Is there are simple ratio involved here - is it predictable? Or are there other factors involved that makes this impossible.

My only other strategy, that I'm employing now, is make a bit of a guess and then removed unfilled wagons after the first delivery of raw materials.