Keller is 2.1 miles south of the County 117 and County 15 intersection. That is the way I go. I go down Bamber Valley Road, County 8, down to 40th St SW (County 117) and go right/west on it. Just stay on County 117 until it gets to County 15. It is not a straight shot to County 15, but it is all 117. There is a stop sign, turn south, curves back to the west and hit 15.
So there are two places to hit 40th St, either at the exit on US 63, or else you can go down Bamber Valley Road (County 8) and then turn on 40th St.