Vacuum Required for Vacuum Advance

[4tiresngas]

In another thread I described how my truck bogs down when I floor it. I thought I would check the vacuum where the vacuum advance on the distibutor is hooked up.

On the side vacuum port on my Q-jet, the vacuum gauge read around 15-20 while idle. When I pushed the accelerator, the vacuum dropped to 0. On the front port on the Q-jet, the vacuum read 0 at idle and went to 20 when I accelerated?

Is this right? and which one should the vacuum advance be hooked to?

Thanks.