Ideally, you want at least two things from any gear: you want it to sound great, and you want it as loud as you need it, when you need it. The same applies to the Digitech RP or any other effects device, and it’s pretty simple in theory. In practice, getting the desired sound at the right volume is about tradeoffs.
There are two tradeoffs you need to make when you set the default master volume for the RP, which you do whenever you adjust the master volume (i.e. turn the rightmost knob when the device is in preset mode, meaning when you’re not editing a patch or in stompbox mode). The first tradeoff is between volume and feedback. In general, you get more volume with less feedback by using the PA to make the device louder, so device master volume should be set relatively low (certainly below 60) as long as you’ve got more headroom on the PA.
The second tradeoff is between bypass mode and preset mode. If you ever use the bypass, which I do frequently, then you want the device to have the same volume when you’re playing presets or bypassed. I find that setting the master volume in the 55-60 range makes the presets about as loud as the bypass, so that’s where I keep it. If you never use bypass mode, you could set the master volume even lower.