Just had my supercharger belt replaced.

However, the supercharger gauge stays around the -10 mark (almost all the way to the left) - and only when I push the pedal down and engage the supercharger does the gauge go up just barely past the 0 mark.

I thought the needle stayed at the 0 mark normally when "not in use" and not around the -10 mark?

-10 is normal at idle. This indicates typical engine vacuum. Under a full throttle condition, you should see about 8psi of boost.