Nearly every strength and conditioning coach in the country relies on percentages to enforce desired intensities to athletes for certain lifts. Basically, the client would perform their lift at a percentage of their 1 repetition max. If they had a max bench press of 100lbs, and you wanted to do 3 sets of 12 reps at 60%, they would do 3x12 at 60lbs. This is absolutely necessary in most strength and conditioning environments for multiple reasons, including consistent program design as well as enforcement of intensity (speaking from experience, some people WILL slack if given the chance). So the question becomes, are percentages right for you?
When presenting your business plan to an angel investor you must understand that they will be very interested in your spreadsheets and proformas, but you must also realize that it is typically an entrepreneurial optimistic approach, which causes problems with proformas. Therefore, you should have dueling spreadsheets; that is to say the spreadsheets, which take your best guess and double the time, double the expenses to compete with your optimistic approach. You should be able to present both of these to your Angel Investor; who chances are is a retired business person with a little bit of financial savvy.