I am no expert, but I have a 304 w/ mc2150 and I run at 15 HG and 12 BTDC.
It runs like crap with the vacuum advance disconnected (as it should) mainly because of the way a distributor is designed to work via ported vacuum instead of raw, and the physics behind ingnition/combustion and the dizzy advance requirments at speed.
I have found that using a vacuum guage to time my motor instead of a light makes me a bit to far advanced (adjusting dizzy for highest HG) and I Ping a bit so I run it at 12 not 14. At 8, I have no umphf......
I have been told that 15 is a bit low on a stock 304 (performance cams also tend to make HG lower) and I have acutally added a vacuum storage canister to assist with my power brakes (which seems to have helped) but my motor seems okay and overall, I run fine with no signs of vacuum leak anywhere. I have not done a compression check to see if I have worn rings, but via vacuum testing, I am pretty confident my valves are okay (with a vacuum guage, you can really test alot of things on a motor related to wear and adjustments)
So to net it out, 15 works for me, and as long as you are running okay and have no vacuum leaks evident, then I wouldnt worry about.
Just ensure the dizzy is around 10-14 unless you are a CA vehicle and trying to pass smog (then it should be 5-8 BTDC) and ensure your dizzy is on ported vacuum. The 304 has a 3 port CTO switch on the manifold. Put it on the center one marked with the D. Some people attach to the EGR port on the front of the carb as well since that is also ported. Set base timing with the dist. vacuum disconnected and Idle at about 800 RPM. Once set, re adjust your carb using your vacuum guage for the higest HG being careful to only adjust outward until the HG level is reached, then go back in until it falls, then back out again just until the higest level is first achieved again. That sets you to the best lean condition and should work fine for you. If it wont idle and you choke is correct, then check your carb internals for wear