52.89?? The band brake force equation?
It is basically the same as the belt friction equation eqn 56.23. Maximum force should be on top which in the band brake problem, your applying force will be greatest. They called it F1 for some reason. Its a little backwards from a belt. The max force will be the driven side of the pulley. Here max force will be behind the brake disk (opposing rotational torque). I just always ensure the larger force is on top.
Im think the force ratio must be >1 for the equation to work properly.