Fix the plate in place.
What the best way to do this is, will depend on what the pressure plate looks like. However the key fact to remember is that the pressure plate works by being depressed by the weight on top of it. So it is not actually necessary to put something else of exact weight on a pressure plate in order to stop it from going offfiring an alarm. It just has to not lift up when the item is removed.
Options:
Super Glue: If the pressure plate's mechanism is open and available to see then try spraying adhesive to lock the pressure plate in place. This is like spraying super glue into the gears of a clock to prevent the gears from moving.
Make a Mold: Create a mold of the pressure plate that will lock its depression in place. This one is a bit harder because any mold batter you pour on a plate will also have weight, but you could probably lift the item by the same force you are adding, since you know how much batter you are adding. The batter just needs to coil around some external object so that it can rigidly be kept in place.
Two tables and a metal bar: If the item is completely in the open, you can place 2 tables on either side of the plate. Adjust the height of tables until they exactly match the height of the plate. Then use 2 rigid poles (cast iron, wood) that cross over the plate and are attached to each table to lock the plates depression in place. This one adds no actual weight since the tables are supporting the poles' weight. The poles just lock the pressure plate in place by preventing it from moving up.
Other similar ideas. The limitations are only how fancy your thieves' gear can be, and the type of scale you are dealing with.
Those are just some basic ideas. The key concept here is to lock the pressure plate in place, rather than trying to match an unknowable weight.