What are you using to shift this rear derailleur? There is no Shimano road shifter that will work with 10 speed MTB rear derailleurs. The only compatibility between Shimano road and MTB is 9 speed and below or 10 speed road shifters with 8/9 speed MTB rear derailleurs.
There is nothing from Shimano in the 10 speed and below range that is designed for anything larger than a 36t cog. A derailleur hanger extension is the only option to make this work.