timing question

You can use a vacuum gauge also.
At idle bring your timing up to max vacuum on the gauge and then back it down 2 to 3 from the highest reading on the gauge at idle.
This is a fast way of getting you in the ball park for advancing your timing past the stock setting.
As far as the timing mark moving,your distributor could be a little worn or the bushing in the block that the distributor sits in could be worn also.
It's very common for the mark to bounce around a little with a stock distributor.
How much the mark moves around ---being to much or the normal amount a stock distributor moves around----is hard to say without seeing it in person.
Always time the car with the vacuum hose off the distributor and plug the end of the hose that's still connected to the engine.
Check the plugs after a test drive and make sure to adjust the carb idle screws also after readjusting your timing so it isn't lean or rich.

I never use my vacuum advance because it's drag raced ----but if it's just a cruiser ,you will save some gas keeping the vacuum advance hooked up.
Keep a ear out for detonation. GOOD LUCK.:burnout: