How precisely are airspace borders defined?
The boundaries are defined more precisely than the chart can show. What you see on the chart is a visual representation of the text published in JO 7400.9Z (or the most recent version of it):


  AWP AZ C Tucson, Davis-Monthan AFB, AZ
  Tucson, Davis-Monthan AFB, AZ 
  (lat. 32°09'59"N., long. 110°52'59"W.)
  Tucson International Airport,
  AZ  (lat. 32°06'58"N., long. 110°56'28"W.)   
  
  That airspace extending
  upward from the surface to and including 6,600 feet MSL within a
  5-mile radius of the Davis-Monthan AFB to the points where the 5-mile
  arc joins a 5-mile arc from the Tucson International Airport, AZ,
  Class C airspace area, and that airspace extending upward from 4,200
  feet MSL to 6,600 feet MSL within a 10-mile radius of Davis-Monthan
  AFB to the points where the 10-mile arc joins a 10-mile arc from the
  Tucson International Airport Class C airspace area.


So in theory, if you're at 5 miles and 1 foot from that lat/long point then you're outside the class C's inner area. But the chart has to show that information in a way that's usable for practical purposes, which means that there has to be a visible line on the chart and that line has thickness. It looks to me like the airspace boundary lines on the chart are roughly 0.25nm or about 1500ft thick, and I have no idea where the boundary really lies on it. You simply have to consider that as a 'margin of error' in this case and plan accordingly. If in doubt, definitely contact ATC.

As for getting in trouble, 1500ft is probably too close for comfort for a manned aircraft, and ATC reacts on what they see on their radar, not what you see on your GPS. It's an interesting aspect of this question that you can probably be more precise with a UAV than a manned aircraft, and 1500ft might seem like more than enough buffer for you. But again, if you have any doubts or questions I would ask ATC directly.