Mainstage 3 mapping midi expression pedal

Mainstage 3 question. I have recently reworked my guitar rig to include some pedals in a rack drawer that I can control via my midi controller (Voodoo Lab Ground Control Pro). I decided to add my M-Audio M-Track to the rack so I can use virtual effects and plugins with MainStage. Everything is working pretty awesome so far and I'm getting some really great sounds without my full pedalboard so it will make for a backup/rehearsal/smaller rig.

One of the patches I am working on now is going to be for big ambient swells. I'm using the Valhalla Shimverb plugin (great plugin, especially at that price!) and I'll probably add some digital delay to it as well.

I want to use my expression pedal (Ernie Ball Volume Pedal Jr.) through the expression jacks on the Ground Control to control the volume for this patch. I got that to work with a little trial and error. The problem is that when I use the expression pedal it is controlling the overall volume of the patch. I want it to control my input level so that when I lower the volume with the exp pedal the reverb and delay continue to ring out. I see a setting in the Assignments and Mapping tab labeled "input level". I think this would be the parameter I want the exp pedal to control. The little icon next to it has a lock on it and I can't adjust the parameters. I'm thinking this one is set up for more like a toggle switch rather than a signal that sweeps across a range.

So basically, I want to control the volume of my patch before it hits the plugins/effects. Is this possible? It seems like it shouldn't be this hard to make it work. I've tried Google with no luck so far. Here is a pic of what I have so far. When I select "volume" then I can adjust the parameters. When I select "input volume" I don't have any options to change anything.

Hopefully what I'm trying to do makes sense. I feel like it really shouldn't be this difficult. Thanks in advance guys. I really appreciate it