You really need good quality belacan / shrimp paste. The below par ones are not worth getting ... just a bunch of salt and what else, I dare not mention.
Unfortunately, there are no exact measurements for the ingredients of this recipe. It all depends on the heat of the chillies and your tastebuds. The end result should be a good balance of heat, saltiness and tartness. Some people prefer a bit more of the shrimp paste taste (the result will turn out darker) and some may just want a hint of it. So adjust as you go along... buzz, taste, add, buzz, taste ....