I'd be interested to know what other people consider a fair range, but I don't worry about it if my coin comes close to the stated weight. I feel like there are going to be tiny variations in almost any case.
Any type of wear can affect it, or even how far into the run your coin was minted in.
This thing will only stop air... to get accurate weight.. , but still coins will have variation in weight.... there can be many reasons normal wear, dust, during punching in machine , there can be errors the strip from which coins are punched etc... etc..