Timing with a vacuum gauge

Hi all,
I've been trying to time my 318 after I put on a new carb and intake but I think that the balancer has slipped or something because I can't even see the timing mark with my gun.

I've seen some information about timing an engine with a vacuum gauge on the internet so I gave it a try. I adjusted the distributor to max vacuum (about 22) and then brought it back a little to about 20-21.

My question is have i done this correctly? Is this an accurate enough way to time the engine that i can move on to tuning the carb and stuff? Or do i need to find another way to make sure I get this right? Thanks!