When we started our Biz six years ago we decided that a safe way to run our finances would be to take 33% to run and reinvest, 33% set aside for our Taxes and 33% as Income. We run solo (my wife handle the books, I do the customer service and work) She also works clerk handling the money accounting for a supermarket. I am also retired from the same company and collect a pension from it. (so now this is my full-time gig). We use these percentages to keep me from over spending on tools we may or may not need.
I know that when you first start you may feel that you must invest money to make money. True to a certain extent. We have done this formula since the get go and it has seemed to work for us.