I've been keg-priming lately to avoid going out more frequently for supplies, and to get around the purported/potential CO2 shortages. I have found that the BeerSmith calculator is pretty good, *if* you are filling up the keg completely. If you have a lot of headspace, things can go off the rails pretty quickly, and you'll end up undercarbed. I used 3 oz. on an American pale ale recently, and it ended up just about right. But...the keg was filled within an inch of the top.
When I do my calculations, I aim for the upper end of carbonation for the style, so that even if I have problems I should hopefully end up in the ballpark. I have (so far) found that about 10 days seems to be the right amount of time to hit full carbonation (sitting in a garage at about 72 degrees for the carbonation).