Jim,
I don't think you will ever find a defined distance from a tower. Being that they are ground based transmission towres, there are lots of variables like terrain, obstacles, interference, tower height and many more that will vary the distance.
As an example, prior to the ADS-B tower going live in Cedar Hill, TX I could fly at 4,000 ft agl and I could pick up the ADS-B signal from Houston and Waco. I think it is very fair to say that you should see coverage within 70 miles of the border, though I suspect you won't find a commitment to it. You can also look at the website www.faa.gov/nextgen for more information. They have a great interactive map and coverage feature.