most of your turbo cars have vacuum leaks then
usually -18 to -22 is normal
if it shows -10 then you have vacuum leaks (more than likely straight of the intake manifold) or (happened to me) bhg... altho that's not common i don't think
mine was surging hardcore like it had a major boost leak
here's some good info from
www.cygnusx1.net
http://www.cygnusx1.net/supra/techtip/vacuum.html