Now add 2 Tbsp. fish sauce, 1-3 tsp. or more red chili sauce, depending on how spicy you want it, and 2 1/2 to 3 Tbsp. brown sugar.
Note: This may seem like a lot of sugar, but you need it to balance out the sourness of the tamarind paste. This balance is what makes Pad Thai taste so wonderful!
If you're not sure how much sugar to use, taste-test the sauce - note that it will be very strong at this point (the taste will be "watered-down" when distributed over the noodles). Set sauce near the stove for use later.