Try -12db to -18db. This is the same as you would set for outputs in the studio to DAW. Some people like it a little hotter than that. But clipping means you are too hot.
Actually there's no rule of thumb, because usually inputs of active amps have got a Gain (or Sensitivity) pot that will actually change the input signal.
Has your cab got one?
So,
Haven't used the Kemper in a long while - plugged into my Gemini 1x12 powered FRFR and my old settings seemed to be clipping the input of the speaker.
Is there a rule of thumb for the sweet spot for monitor line out with current OS to a powered cab?
I know I can noodle with it but thought I would ask.
All good answers above.
How could your old setup, that used to work, not work now? That is a different question than the one you asked. I would be more curious about that.
There are too many potential variables regarding gain staging to provide a single answer so noodle with it. IMO, just turn some knobs until it sounds good and get on with it. 
