you should can work out the torque generated at the output of the gearbox in each gear right through the rev range. you need a print out of the engines torque curve.

please correct me if I'm wrong but its normally an advantage to hold each gear as long as possible. (N.A. cars)
so change gear when the output torque in the next gear is greater than the output torque in the current gear.

example.
in 1st you might have output torque at 3000 rpm of 1500Nm.
in 5th at the same rpm maybe 500Nm.

I hope that sounds ok.