Just enter new favorites in different size servings. For example, my almond milk nutritional info is 1 serving = 1 cup. What if I like to pour a small splash into coffee? I re-entered (and saved) a new entry using "1 serv = 8 oz" since 1 cup equals 8 oz, right? Then I can guess my splash to be 1 oz or 1/4 oz (or I could resave info with however many Tablespoons per cup.). It would work with metric too.). Makes Life Easier!
As much as possible, I try to enter serving sizes in different amounts and let SP calculate the calories per unit. I figure if a cup is a known amount, say, 640 oz per cup, then I'll know 1 oz is 80 and so on. Some people are blessed to do this in their heads, but I am math-challenged!!