Mighty Gem,
The C of G is an approximation for all aircraft, since the critical flight maneuvers that establish the limits are based on flight tests where the cg naturally moves with fuel burn. For the S-76, we try to flight test within 1/2 inch for critical maneuvers, but 1 inch is allowed (out of a total range of 17 inches). We often start the tests 1 inch outside of the limit, and burn to 1 inch inside the limit. The 1 inch is therefore about 6% of total CG, quite within the common sense range of variability of flight maneuvers, considering all the things that influence the behavior of the craft.
I would take as a rule of thumb that 5% or so is the likely accuracy for CG determination, so take your aircraft's CG range and divide by 20 to yield the likely tolerance band for such variables as whether the pilot slouches in his seat, or if the fuel is 6.3 lbs/gal instead of 6.0, etc.
None of this should be construed as encouraging a lax attitude concerning limits, but rather as showing that basic rules of thumb are acceptable for the calculations that we all depend on.