Wow, looking at their page and documentation they don't give much detail for expected input values.
I would turn off the Fryette and set its volume to 1/2, turn on the modeler and set its OUT volume knob to 0, plug the modeler in, turn on the Fryette, let it sit for a minute so it finds its happy place, then play something on the guitar (assumedly it's plugged into the modeler) and turn the OUT volume on the modeler up
slowly until it's the volume you want
without distortion. Repeat playing something and adjusting the volume as necessary. If it doesn't get loud enough, increase the amp's volume a bit and continue adjusting.
By slowly adjusting the volumes you can find a nice balance between the modeler's send and the amp's receive settings.
Back when, we'd plug all sorts of stuff together, and that was the general process, start with a reasonable setting on the receiving side, and nothing on the sending side, then slowly turn things up. It's how we'd do FOH for small music festivals when mixing together multiple different PAs.
Oh, and we had soldering irons, a variety of different plugs and a sharp knife with a screwdriver.