I have sort of the same issue when I drive to SPI the GPS wants to send me in a much more southernly route when I prefer the major highway through Oklahoma City as I drive at night as opposed to coming through Muskogee. I set up waypoint just outside Oklahoma City then route through to SPI. I know with my eTrex I can set it up on my computer and transfer to the unit. I recently got a Nuvi 650 and as yet have not tried it on the same trip, however, I anticipate just having to first go to Oklahoma City (or close to it) THEN insert my next waypoint for SPI. While one "could" set up more complete routes on some of the older GPS units (eTrex of Garmin-18) fact is my wife found them very confusing and couldn't really use either one. True the Nuvi is dumbed down but my wife can now use the unit and I've heard her say that she really likes the "GTO"(?)
with that take me home button.