I used to be a professional barista and this guide is not bad, if you really want to be anal I would say that the air pressure can effect the grind also so take that into consideration. A great resource for this and more importantly correct milk steaming is coffeegeek.com
Now if only people like you and I worked in high street coffee shops I could drink coffee again
Oh one other thing, using a spirit level, especially the round ones, is a good gauge of whether you are tamping properly and evenly. If you tamp one sided like a lot of people do the water will filter through that part of the grind more easily and leave you with half used coffee and a potentially weak espresso.