I generally make my curries in double portions. Occasionally, I cook three portions in the same pan, but I rarely make curry for one.
300ml of base gravy seems to be commonly accepted as sufficient to cook a single BIR curry. I use 450ml for a double portion.
For spices, powders and wet pastes (such as tomato puree etc), I generally use half as much again, i.e. if a single-portion recipe calls for 1 tsp or 1 tbsp, I'll use 1.5 tsp or 1.5 tbsp.
For meats, rice, breads etc., I simply double the portions.
There's been lots of debate about this subject, but using half-as-much-again for everything except the meats and accompaniments works very well for me.