What is the distance at which my VHF communication radio can reach ATC?
The TKM MX11 has an 8 watt transmitter which makes it a Class 4 transmitter as defined in RTCA DO-186B MOPS for Airborne Radios Communications Equipment. That specifies a maximum range of 100 miles.  

In an area of relatively level terrain, you should see a range of about that 100 miles if you're at 5000 AGL or above. Below that, range will be reduced due to the curvature of the earth and multipath interference. At 2000 feet above level terrain, 40 to 50 mile range is typical based on an analysis I did about 20 years ago for a former employer.  

One factor you can't control is the height of the ATC antenna.  The higher it is, the greater the range. Even a 50 foot difference in the height can affect the range by several miles.