Well whatever speed, you should not have to use just 30% to 50% of the power to go slow and keep from boiling over. There really is something wrong if this is the case. If this has been going on since the last cooling service consider a different shop. Need someone that can find the issue and repair. Anybody can swap most of the coolant, say they changed the thermostat and checked the fan and send you down the road. You need someone to really get in there and fix it.
240-250 should be top of normal with 260-280 is boil over range. I assume it should run 200-210 max on the flats at 65 mph.