shezza999 yes that's spot on - and you will get this result with any curry, Madras or not. Some folk advocate the two stage reduction process:

stage 1 - half portion of base (say 150-200 ml), reduced rapidly with spices, tomato puree, garlic ginger paste etc.
stage 2 - half portion of base, reduced till sauce is the desired consistency

Thus you can expect a starting quantity of say 400 ml (i.e. Taz base) to be reduced to say 250 ml for a single curry. This is no different to classic sauce reduction in say, French cookery, where the thickness of the sauce is achieved by evaporating off some of the water content (reduction) as opposed to trying to thicken a sauce by adding cornflour.

Hope this helps.