Yes, you can make a salad dressing with tofu. To figure out how much of this homemade dressing would count as your fat serving, you could use a tracking program to calculate the nutritional information. This would be complicated, but once you do it, you’ll know for next time. One serving of fat typically has around 120 calories, so aim for a quantity of dressing (perhaps 1.5 oz., perhaps 2 oz.) that approximates 120 calories, give or take.