Tyre - also free is much simpler to use than BaseCamp and will transfer routes to Garmin. The only way to make sure the GPS unit will follow the exact route that you want is to add in waypoints making the GPS follow the route - in other words if the route you want goes left and at that point the GPS wants to go right, go back to the routing software and put a waypoint on the road to the left.
My GPS will stand on its head to put me and my RV on routes it is not allowed to travel on unless I am very exact in the route with waypoints along the correct roads. The only way to know this is to load the route into the GPS and review it and then go back and make changes in the map software to force the route in the GPS.