billycat,
i always use 300ml to produce a 200ml finished portion. my ladle is 150 ml so i put 1/2 ladle in after the tom puree/spice mix and fry it off, then 1.5 ladles and simmer until it starts to crater or i can see oil (whichever happens 1st).
i've been using my take on real BIR base for the last few weeks trying to perfect it. i can post the recipe if it's of interest. i am sure any good base from the site would produce the same result ie saffron or rajver being the ones i use on regular basis.