The professional way to write a cosmetic "recipe" is as a formula which is written in percentages. This is the way we teach in our formulation course (Diploma in Natural Skincare Formulation, Diploma in Natural Haircare Formulation etc). From this you can convert the formula into any weight you like when you come to actually making the product (ounces, grams, etc) either using a simple calculation or one of our batch calculators which does the calculation for you.
We have this blog article which may help you: www.schoolofnaturalskincare.com/how-to-turn-a-cosmetic-formula-into-a-recipe/
When measuring we do advise you to measure in grams as this is the most accurate and professional way to measure ingredients. We recommend you use digital scales that measure in grams to make things really easy. For formulating you will need scales that measure to 0.01g.
Our Certificate in Making Natural Skincare Products is different as it is not a formulating course. This course contains many recipes for you to practice with and adapt if you wish. These recipes are all presented in grams and each recipe equals 100g. This is so if you want to convert it into a formula it is easy to do so. If you took this course we would recommend that you use digital scales that measure in grams to make things really easy. Alternatively you can use free online tools to convert the recipes into ounces if that is what you prefer.