Fair enough
However, (there is always a 'however'!) you are talking about reading accuracy and instrument error. In addiiton there is also, for want of a better word, experimental error, which is effectively how accurate the point can be flown in the given conditions. Again, based on experiecne with test pilots, the GPS variation during a given run was between 1-2 knots, and the airspeed variation was ±1 knot. All this was captured on flight test instrumentation, which gives a better picture tha just a simple snapshot which you have probably done. Please note I am decrying your attempts, at both flying skilss and anlaytical skills, I am just trying to pass on some some real life experience.