... or " I did all that, why are my drop numbers still not true ? "

When truing the ballistic calculator to the actual drop, it is common to tweak the BC or velocity number to get the calculator to produce a number you are observing in the field. That method is certainly going to cause problems for you if you can't shoot well enough to make the determination at range. And by changing the BC on the calculator to true at a specific range means that you intorduce a margin of error when the range changes, if the BC number you use is not actual.

Truing the calculator by observing impacts at range only works as well as conditions permit you to shoot, therefore the margin for potential error is significant. It is very likely that you true the calculator this way for a specific range, and when you decrease or increase the range, your drop is not accurate anymore.

Ideally, you should not have to change the BC number, only a slight change to the velocity should be enough to true the calculator.

If you find that you have to change the BC number on the calculator, you should consider verifying the actual BC of the bullet you are using.

Bullet BC is a calculated average number. The number changes with barrel twist, bullet velocity and atmospheric conditions.

"To a rough approximation, the BC [ballistic coefficient] can be estimated as the fraction of 1000 yards over which a projectile loses half of its initial kinetic energy. In other words, a bullet with a BC of 0.300 should lose roughly half of its initial kinetic energy at a range of 300 yards."

The BC number on the box may not be accurate for your firing conditions. This is because the BC number on the box is specific to a particular velocity range at a stated atmospheric pressure. The manufacturers will quote a BC number that corresponds to the conditions that they think most of their bullets will be used at. It may be that your use case is outside those parameters, hence your inability to true the calculator with that BC number for all ranges.

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.

Once you have that, you should find that the ballistic calculator is now more accurate for you at various ranges.