The only comment I'd make is that the order of columns in a covered index "just depends" , generally the most restrictive ( selective) is best placed first - but not always - so it's worth making a few tests including putting the columns in reverse order, I tend to create a number of variations for a particular query and see which the optimiser finds best ( not for simple queries or all queries obviously ) Also worth noting that changing data volumes can change the index selection process.

Nice article.