My uninformed opinion based on no experience is that this will creep so you should use a Kibble balance instead. (See sibling comment by s0rce.) Or maybe use

My uninformed opinion based on no experience is that this will creep so you should use a Kibble balance instead. (See sibling comment by s0rce.) Or maybe use a lever arm to amplify milligrams into tens of milligrams of force. Or just a regular pan balance. How big a weight do you want milligram precision on? If you have some way to cut a precise shape out of some kind of metal sheet of well-controlled thickness, could you cut out a milligram precision bismar balance or steelyard?