You could achieve adjusting the max parameter on mod wheel to be 50% instead of 100% in a few different ways:
- Go into the instrument plugin and set a new high range max in the mod matrix for instances using midi CC1 (your mod wheel).
- Add an instance of the MIDI Modifier to the channel strip that looks like this:
- Go to the MIDI Input tab for the channel strip. Setup Input Controller to Modulation and Output Controller to Modulation (Unchanged). Then open the transform graph and set it so it looks like this:
Option 2 is super easy and fast, but Option 3 would let you control even more of the mod wheel's response if you wanted.