Sounds like you have the vacuum advance hooked up to ported vacuum. It wont come in until you open the throttle and get to a higher rpm. You stated you get 18 at idle dialing back but also noted setting the light to 0 it was at 12. Use the dialback reading. I have to ask why you're adjusting the timing? If at 18 and you want to try 8ish, is the car running poorly? Is your engine completly stock or built? You want to set your inial first. Diconnected from vac advance can. Once you get the car idleing well, dont change it. Now find out your mechanical. Get your RPM's up to 3,000 and you should be all in with mech. Take that reading. Subtract the inital # and you have mech #. Now connect your vac advance and take a reading at 3000. That's your total. You'll also be able to figure out how much advance the can is adding. The best timing is a couple degrees less from when you start to hear detonation. You dont change inital timing but adjust the springs & weights in the distributer or limit the amount of vac advance on the can. FWIW, I'm running 14 with vac advance connected to manifold for 22 inital. Mechanical is 14, so all in at 36. I had to weld a stop on the vac advance can but didnt change the springs or weights on the mechanical.
