Wherever you purchased the FO from should have the usage rate listed for each of their FO's. They generally list soaps/creams and balms/and lotions, so I would go to them and see if I could find it. If not, I would call and ask them. If they say they don't know, then make sure that you ask if their FO's are skin safe in the first place! If they say they are skin safe, but don't offer usage rates, I would be wondering about the truth of that statement, but if you want to risk it, then I would start w/ looking at the BB calculator, then experiment.
As for making your own, it is certainly your choice and your preference. I personally believe in making your own if you are selling the products, but I see that a good many people disagree with me. As for which is easier, a base requires a bit less work, but making your own from scratch is an education and requires that you be educated in the craft in order to make it. I feel that education is of paramount importance when selling skin care products. So, for me, the rewards of creating it from scratch far outweigh the inconvenience. This is my greatest pet peeve though, and you don't need me getting on my soap box, so sorry
!
Good luck!