Is the chronograph 15 feet from the muzzle giving any valuable information other than shot to shot velocity variations?
It is giving you your best, most easily available muzzle velocity information. What you are actually measuring is the bullet speed at 15 yards (+/- the chrono's random errors and systematic errors). To get muzzle velocity from that, a calculation has to be made (most ballistics programs are able to do this), and while this calculation is not foolproof is it relatively tolerant to small errors.
Is setting the chronograph at 100 meters a more accurate way to determine velocity?
It is a somewhat less accurate way of figuring out your muzzle velocity. The speeds you measure at 100y are just as accurate as the speeds you measure at 15 yards, but the calculation to correct back 100 yards to the muzzle is a bit more error prone (because the correction is larger).
Is the actual bullet drop the most accurate way regardless if what the chronograph says?
No, that is one of the least accurate ways of calculating your muzzle velocity. This method is quite sensitive to very small errors in assumed bullet BC, the group size that the rifle fires, and unless you take deliberately *NOT* made adjustments to your scope's elevation knobs from your (say) 100 setting, the actual size of your scope's adjustments will contribute to the error in this method.
If you aren't actually interested in measuring your muzzle velocity, but *are* interested in determining your sights settings for a variety of distances, there are two good ways of doing this:
1 - if you have easy access to a rifle range that goes as far as the distance you are interested in making your table for, shooting at several distances is the simplest and most foolproof way
2 - if you can't shoot out to your max. distance, you can do a good job by
- carefully measuring your muzzle velocity
- get a good 100y/m zero
- measure your scope's actual adjustment size (measure the _actual_distance to your target at "100y/m", measure how many inches "20 scope minutes" moves your bullet's point of impac)
- calculate your ballistics with a good program (e.g. JBM) using good data (e.g. Litz G7 b.c. values)
- if available, confirm/crosscheck your table against actual shooting at the longest distance available to you
