I know a lot of us have problems trying to decipher which extensions to use on a given tune or chord progression. I found a surprisingly easy way to determine the "correct" extension choices in any given situation. I have used the tunes There is No Greater Love, The Days of Wine and Roses, and All of Me to demonstrate this topic. All of these tunes have a lot of secondary dominants as well as tritone substitutions, perfect vehicles for this topic.