I understand there are formulas to calculate the amount of NaOH required based on the SAP values of various oils, and then a formula to calculate the amount of water to dissolve that in. However, is there a 'rule' for selecting the weight/volume of oils to fit in an x pound mold when creating your own recipes?

I would think it would depend on the lye discount. As well as a small variation in oils. For instance 54 oz of OO will make 63.2 oz where as 54 oz of lard = 63.4 and shortening makes 63.3 I get 1 lb of cured soap from 13.5 oz. of oil with my recipe. I use a 10% lye discount. I don't know the formula but I have spreadsheet that does it for me. I started out with a 7lb recipe and had to tweak it for my 4 lb mold.

Just use this online calculator, I love it! It will calculate everything for you. Just know how many ounces of oils your mold needs. That is figured by taking the length times the width times the height you want to pour, times .40. This number is the amount of total oils, butters, or fats you put into this calculator: