I would check total advance first. Pull and plug the vacuum line to the distributor (if equipped). Set your light at 35 degrees. Have a helper rev the engine to 3000+ RPM and set timing so it flashes at the zero advance mark. That will get your total set at 35 degrees.

Next, slow engine to idle. Do not move the distributor since its already set for correct max advance. Set the advance light to zero. Check timing with the factory marks. If idle is OK and advance is about where you want it you're done. If not, your distributor may need tweaked some more.

Note you could use the advance function of the light at idle or any other RPM. Adjust the advance light so it flashes at TDC and see where the advance setting lands. That's your timing. I just think its easier to set the light at zero and use factory marks as long as they go far enough.