Or just be specific and say you want x number of bird's eye chillies in the dish if you're having Thai food. That'll do it. (When my partner and I were in Thailand, every now and then a vendor would ask how many chillies we want, which I thought was a good idea, because saying mild, spicy, or very spicy leaves quite a bit of room for interpretation. I find bird's eye chillies tend to be fairly consistent with their spiciness, unlike some other types of chillies, so that worked well. Though you do also need to have a rough idea of the approximate size of the dish, of course.)