Does your distributor have the large Hex Nut looking fitting where the vacuum line connects? Those units will un-screw off the pot housing and there are shims there to set the spring preload.
I've got mine at 7 degrees full vacuum can advance, 26 degrees of centrifugal and the initial at 10. That nets an idle advance of 17 and the stock cammed engine likes that just fine.
Steve Metzger Tucson, Arizona