... or trouble fitting to the curve.

There is a simple way to get a reasonably accurate actual BC number, using a chronograph, and the ballistic coefficient calculator at JBM Ballistics.

Measure the velocity the usual 5 or 7 yards from the muzzle. Then move the chrony downrange to a point at which your ballistic calculator says your velocity has dropped by at least 5%. If you can setup at the 10% mark, also good, but it is not necessary to go further. Measure the velocity at that range. Measure as precisely as possible the distance between the two chrony positions. The most accurate way to do this simply is with a tape measure over a level range. The accuracy of this measurement determines the final accuracy of the calculated BC, so pay attention to it.

I suggest using a five shot average for the near and far, so 10 rounds in total. This will help to iron out the velocity variance, giving a more reliable BC number.

It is critical to use the correct data for elevation and air temperature. Measure these on the day, at the time you are testing. Plug your measured data parameters into the calculator, and it will give you your actual BC number for those conditions.

For more background information on determining ballistic co-efficient using a chronograph, you can refer to a more technical write-up at ResearchGate.