This is my most recent formula:
12 2-oz bottle recipe, goal was 25 oz total
17.5 oz distilled water
3.75 oz sunflower oil
1.25 oz shea butter
0.75 oz stearic acid
1.25 oz emulsifying wax
0.25 oz fragrance
1.875 oz Germall
I added 19 oz of distilled water instead of the 17.5 in the recipe. Unfortunately I only wound up with approximately 21 ounces of lotion total. Is there a general rule of thumb as far as how much extra distilled water I should add, in order to get my desired amount of lotion?
The issue here isn't that you haven't compensated for the evaporated water. The issue is that it's hard to figure out the volume of lotion you'll have based on a weighted recipe. For instance, this appears to be a 65% water recipe, which means that you should get at least 17.3 ounces by volume of the product, but we can't assume much more than that.
Doing some calculations, I notice you have 26.625 weighed ounces in this recipe with 17.5 ounces of water, which means this is a 65.7% water recipe. Even at 19 ounces of water, you only have 67.6% water (26.625 ounces plus 1.5 extra ounces = 28.125. Now 19/28.125.) There's no way this recipe could total 25 ounces by volume because it has a ton of solid ingredients, like stearic acid, emulsifying wax, and shea butter. If you want more lotion, you can do one of two things. One, you can make more of it. If you ended up with 21 liquid ounces when using 28.125 ounces of weighed ingredients, you can figure out the ratio and make it that way. (This recipe works out to about 75% volume by weight, so to get 25 ounces, you'd want to make about 33.5 ounces by weight.) Or you could make a different recipe and go with a larger water amount. The more water, the closer the product will be to the weighted ounces. If you are using an 80% water recipe, it's going to make more by volume than a 75% or 70% water recipe. But there's no really good way to estimate how much a recipe will make except for making it. (Which is one of the reasons I say to get into the workshop and make things! Experience is a great teacher!)
This is so confusing - ounces for weight and volume? The metric system is so much easier!!!
To compensate for the water that evaporates during heating and holding, measure your entire container for the water phase before you heat it. Boil up some water and let it cool as you heat and hold. When you remove the heated water phase container from the double boiler, weigh it again. Make up for the difference with the water you boiled in the kettle, which should be around 75˚C to 80˚C by now.