You might try to press the [PERFORMANCE] "(HOME) " button first then use the sliders to adjust relative levels of the two PARTs. If this works or not depends on how FC1 is mapped. It can be mapped to control volume in different ways - so there are multiple ways to accomplish this depending on your starting point.

If this does work, then you'll want to [STORE] the result so you don't lose the edits. If you use scenes with mixer memory on - then you may need to also re-save the scene with the new levels and then [STORE].

Thanks Jason.

I discovered in Edit mode, under Mod control, a ratio parameter for FC1. This seems to do the trick. I give a little more to the sampled strings and less to the FM.

Another issue I'm having, which is addressed in a different thread, is when I change patches in Live Set, the strings are on, even though the FC1 pedal is all the way back.
I'll look for that other thread.

I found that thread and you already addressed the 2nd issue!
I've set MIDI Controller to "Hold". Hopefully this doesn't mess anything else up.