I make a lot of curry goat (as the Jamaicans call it), and find that in a recipe using around a kilo of meat for four people, then two Scotch bonnets/habeneros gives a good Madras hit.
It's by no means an exact science though, as not all chillies are equal when it comes to heat (you can even get heat variance from chillies that come from the same plant). I've had some scotch bonnets that have blown my head off, while others have been relatively mild.
That said, there are several ways you can get that lovely fruity Scotch bonnet flavour without too much of the heat. Firstly, you can remove the seeds and the pithy membrane (which is where most of the capsaicin is concentrated), and secondly, you can make the curry goat the day before. The intensity of heat mellows considerably if the dish is left overnight.