Min/Max PWM on outputs only available controls?

I have written a custom mixer for an airframe and got it to work. It needs some fine tuning of the servos to get optimal geometry. Specifically I need to set the min/max and neutral positions for PWM outputs. I can do this on the RC inputs. There are parameters to do so, but all I see for the PWM outputs is min/max that applies to all 8 channels. Is there a way to set these individually or are you expected to scale the mixer outputs to include the extremes and neutral offset in the calculations? I can do that easily, but I just want to make sure that is the desired method.