[BillC]

The c=6 is the "optimal f" suggested by R. Vince (amazon him and see our paper on bjmath.com).
As in poker, a lot of the time investors don't know their edge and variance with much accuracy, so they make guesses and go by the seat of their pants. Marc I. derived a formula for estimating c (giving confidence interval for c -- it is quite complicated) from historical data. This formula could be used for poker too. I will try to dig it out sometime and write it up.
Another comment: It is absurd to use a bankroll that can be replenished. You have to include future contributions to your bankroll. Include the net present value of all future inflows minus expenses and taxes.