Usually the initial is with the vacuum disconnected.
BTW, I'm more a fan of experimenting with timing. The specs are just suggestions. As long as you can hear the knock (pinging, marbles in the coffee can, or whatever it sounds like to you), I'd advance the timing and see how it runs. If you have loud exhausts then stick to the specs. I had a Chevy S-10 pickup through the 1980s and 1990s didn't own a timing light. I just adjusted timing to get very light knock at wide open throttle. It ran much better when advanced from the factory/dealer tune-up settings.
(I did some laboratory work with a Chevy 350 in the late 1980s also:
https://www.sae.org/publications/tec...ontent/890161/)