The Tune Drive is not an overall drive. It's just the level of the tone when you press the tune button, any slice.
The slice drive is per slice, and not limited by the tune drive.
In my case I run the Tune Drive at ~160 to get 10 watts from my first stage amp.
I run my slice drive much higher to make contacts (170-200+, depends on the band).
If you have an amplifier that needs more drive on 80m you can have that slice at a higher drive, or if you are monitoring with FreeDV you can keep that at 0 since you are only monitoring.
Think of the slice drive as extended flexibility. If you only run 1 slice is is not needed, but increases flexibility
73, KD7HGL
Josh