I have seen different quality (strength) of #64 rubber bands. This would make ANY formula just a starting point. One thing you do not want to have happen, which I have seen, is to pull too hard in a maneuver and have the wing lift off the saddle(one of the few reasons for bolts on trainers). The possible change in wing incidence can cause a MAJOR change in handling briefly, enough to lose the plane. Your instructor should be able to show you how to check for sufficient rubber bands. BTW, if your plane can't carry 2 more rubber bands, it has other problems that need looking into. Lets get real IBrakeForNobody, we are talking about a trainer. Who knows what he is going to put this poor plane through.[:@