Here's a good link. http://www.tastybrew.com/calculators/
Different styles of beer have different levels of carbonation. The 30 psi rule works for the majority of your ales at room temp. When you set your pressure on your regulators you will hear gas hiss through your regs and into your keg. When you rock the kegs your agitating the beer allowing for more gas dissolved into the beer. After rocking about 10-15 minutes at 30 psi the hissing will stop because you've reached that pressure and the beer cant dissolve any more gas for now.
As for your Scottish ale, the settings I got from the tasty brew calculator was 4.4 psi. at 68°. That means set your reg to that pressure and rock til you don't hear the hiss anymore. In my opinion that sounds low, but I'm not a computer and haven't done much with Scottish ales. In my past run ins w/ this style however the first thing I noticed was the low carb levels.
Some mistakes I've made that I can pass on...
- Once pressure is achieved disconnect from gas, Don't leave on at active pressure. You'll end up w/ seltzer.
- You can either refrigerate or keep at cellar temp. Refrigerate carbonates beer faster, but some beers like to condition after carbonation for a while. I'd go cellar temp for Scottish ale for at least 1 month after carbonation.
Hope this helps.