I only have two coins that were slabbed and I removed them from the cases as soon as I got them. I never thought to weigh the slabs. And the coins I
had were not weighed by NCG that I know of, I weighed them myself.
Anyway, there are a couple ways to look at this. One is the
weights of the cases plus the various sized holders and how accurate you can be with those. If those are known, one could get a reasonable
weight for the coin. Your numbers that you have come up with so far seem reasonable and would benefit from a larger sample size.
Here is where I am going with this. The biggest question is what level of tolerance are you going for? How accurate do you need to be and at what point would being off make a difference. Many
ancient coins have what I consider a large variance in
weight. More than I expected when I first started collecting. So, I think that for many coins, a method of weighing the entire slab and subtracting what the estimated slab
weight minus coin would be acceptable to many (most?) people.
It really depends on how accurate you want or need to be. I wouldn't do this if I were
selling a coin and describing a specific
weight, mainly because I am strict about tolerances for things like this. I would want to weigh the coin by itself. That said, no one calibrates
scales any longer and, as an old Army calibrator who calibrated
scales in commissaries, orderly rooms where people were weighed for the fat boy program, hospitals, and nuclear/conventional
weapons storage sites (where weighing projectiles is done regularly to determine deterioration of the weapon), I haven't trusted a
scale in years. With a coin, maybe I would say approximate
weight or something, plus I may be the only person in the world who cares. Yeah, I know they say digital
scales are infallible. I don't believe that. No one calibrates voltmeters anymore, either.
And although it is not really true, the inaccuracy in things weighed in grams is going to appear much more than with something weighed in kilos.
I sure don't trust my
cheap China made digital
scales any more than the one that weighs the meat I buy (other than I am cynical enough to believe the grocery
scale is always going to be wrong in the store's favor). But, that is what we have today. If you can get a
good range of
weights for slabs with various sized inserts, it is probably OK, although it introduces an additional element of error. And there will always be at least some error.
Virgil