After coming from a Nikon Coolpix 5700 with a teleconverter, I'm so used to being able to shoot macro to 420mm with just one camera that I really had to do a LOT of research to figure out which two lenses would fill the most of my shooting. In the warm months, I do 99% of my shooting from a kayak where space is at a premium and fighting against the bag with lunch, snacks, etc, the water bottles, the emergency kits, the extra clothing in case I dump in, etc etc. I've become very adept at packing that kayak perfectly and keeping everything in grabbable order during a 12 hour day out on the water. I HAVE to keep equipment to a minimum just because there's no room! *LOL* Going from 17mm to 400mm with just two lenses, and macro with the 500D works.
I do like Heiko's method though. When I learned to golf, my husband made me play the whole round with just two clubs, so I could get a feel for the range each club would produce. Works with lenses too.