The Master and Lead Master on the amp are actually pretty early in the chain, and control the level sent to the effects loop. Therefore, they kind of work best to control the balance between the Rhythm and Lead channels, and your effects send level. If you've got a rack effect in the loop, the MASTER will control the level it sees, which might overload some effects, or under feed them depending on the level they expect to see. Around 5 is the standing mid-level for effects send and probably a good starting point.
After that, I don't really have a good answer. You can set the power amp volume high to ensure it's running well, then use the output level to control volume, (though if you use the effects send straight to effects then the power amp, rather than back to the return then the real outputs, then those knobs won't do anything). I would probably set the output around half as well, then control with the Power amp volumes. But experiment, The output level will control what the input of the Power amp sees, if you boost the output to 10 you might get some additional overdrive at the input stage of the Power amp, or the phase inverter. You might like that, or you might find a low output sounds better.
As for the Effects vs normal, The effects return goes through an additional tube and some circuitry, so it'll sound a bit different than the effects send signal anyway. Then the recording outs circuit actually does do a bit of dampening on the main outputs as well, might take a little high away?
So I'd just experiment and see what sounds good.