CrateCruncher, you are correct. Above stall, induced drag is inversely proportional to the square of airspeed, and is relatively unimportant at several times stall airspeed. For example, at five times stall airspeed, induced drag is only about 1/25th as great as at near-stall airspeed.

Here is a NASA blurb on induced drag that may be useful:

http://www.grc.nasa.gov/WWW/K-12/airplane/induced.html

Measuring drag is quite a tough nut to crack, unless you can get ahold of a wind tunnel, and test a reduced scale model of your airplane. Another way to measure drag would be to use onboard airspeed telemetry, kill the engine, dive your airplane straight down from high altitude until it reaches its maximum airspeed. This may produce a dangerously high airspeed and cause the airplane to break up from flutter, turning it into an unguided missile. The drag of the stationary propeller can be calculated reasonably accurately; a windmilling propeller produces tremendous drag that is also difficult to estimate. At maximum speed, vertically down, drag is equal to weight.