Weight Range For Detecting Counterfeit Silver Coins

I've weighed a lot of US silver coins, and found some as much as 8% below the specified weight (that was on a 1916-S half in AG, with very little detail left). The dollars don't seem to lose as much weight by % as the smaller coins, due to their thickness. Yesterday I ran a specific gravity test on a 1926-D Peace dollar, which is in VF and weighs 26.56 grams, or 0.64% underweight (the specific gravity checked out OK for coin silver). To get to 1% weight loss you'd probably be at Fine grade.

I have been buying a few lower quality, common date Peace dollars and came across one which was about 409 grains rather than 412.5. My scale has 1 grain resolution. With the slippery details on the eagle I was somewhat suspecisus and wondering how much under weight would be a tip off as to this being a counterfeit. My other Peace dollars are spot on 412 gr, so I was suspect. I was just unsure if being 1% under weight was a give away. Thank you thq for your input.