The variables on peanut sauce are numerous. Are you using sweetened or unsweetened peanut butter? That will affect how much sugar you add. How spicy is your chili? If you or someone you're feeding is sensitive to heat, start by adding half of the diced chili, then add a little bit more, as needed.

The thickness of the sauce is a matter of personal preference and also how you plan to serve it. As a dipping sauce for Vietnamese fresh spring rolls, you'll want a medium thickness - thin enough to get the rice paper rolls into the mixture, but thick enough that it sticks. Add more or less water to reach your desired consistency. If your peanut butter is feeling especially lumpy, adding hot water instead of cold may help break it up.