Wow, I'm new to the licensed side of GMRS also but those ranges seem awfully short. From the top of the 10 story building you should be able to get 18 miles according to the "Line of Sight" calculator ( http://www.calculatoredge.com/electronics/lineofsight.htm). Hand held to mobile mag mount should be should be 5-6 miles if nothing is in between. I do it all the time with 5 watts over average forest and farm terrain with a pair of Terra walkie talkies with 15" antennas. There's a spot (with both of us at ground level) where I talk to my friend after he gets off work a solid 12 miles away with MURS, walkies with car mag mounts...can't do it on GMRS though. What you're trying to do sure seems possible. maybe others have the solution. DH