1. Depending on the type of miso used, the amount of sugar may need to be adjusted. If you use Saikyo miso (white sweet miso), you do not need to add sugar. If you use brown miso, then you need to increase the amount of sugar to 1 teaspoon (see the recipe for Squid with Cucumber and Wakame Nuta). It is also dependant on your preference.
2. It is ideal to use karashi (からし, Japanese mustard) but hot English mustard is also OK.
3. To check the consistency of the sauce, scoop the sauce with a spoon and try to drop the sauce into the pot. It should slowly drop like cold gravy. If it is too thick, add some water.
To cool it down faster, put the dressing in the fridge.