vacuum advance distributor help

I have a msd vac distributor with vac advance canister.The canister is non adjustable.The engine has 15" of vacuum which causes the canister to put in an additional 20 degrees timing where i only want about 10 degrees.Do they make something that you could put in your vacuum line going to the distributor to restrict the amount of vacuum to the canister that would in turn put less timing in.Thanks