Setting up a new boat with Garmin 7612 and GMI20's

How do I map an Exp value, say "VMC%" to user channel 1 ?

When I go to Settings/User channels/   I get the window with User 0-31 then
Type (a drop down of types)
Precision (is this damping %)
Name  (seems to take anything I want here and the label appears in the Number Box)
Equation (no idea what goes here???)  does this depend on what system you want the data to ultimately go to?

When I put a name in to User 0 then assign User 0 to a Number box the name presents ok
but I can't seem to get a real  value, say VMC to work  (with simulator on) 

Using Settings/Alternating they all work fine.   (with simulator on) 

TIA
Ernie