You probably would need a "plate" to put the things on that would have a fixed contact surface against the screen (i think there might be some confusion between plain pressure and pressing more of the screen at the same time that the software (or perhaps even the hardware) can't discern)


I dunno what the range would be, but things that are too light won't even register, and things that are too heavy will go "out of the charts".

Also it might be necessary to incorporate accelerometer data to correct for slight misaligments in relation to plain horizontal (for example caused by the lens cover rig, or by the surface the device is on not being perfectly level)

edit: perhaps the scale program would have an option to define the estimated contact surface of the plate, so people could use different things; also, an option for calibrating the scale using weights of known masses (like those labeled ones used on those old balances with a counter balance)