That carbing chart... Yes, useful info for the scenario. But there is another way. After it's fully chilled use the shake method with regulator set at final "set and forget" pressure. Sure the beer won't accept the CO2 as quickly as if a higher PSI was used, but it isn't really possible to overcarb either. Personally I'm usually just patient enough and wait for it, and it gives anything that made it through the transfer time to settle.
As far as head retention goes, it's possible the chocolate killed it. My first attempt at chocolate stout had the same headless fate.