OK I decided to go ahead and draw the thing.
So the idea is. When a measurement needs to be made, the arduino will spin the motor up to make sure the counterweight is all the way in to it's home position.
It will then slowly move the counterweight away from the pivot until it detects that the platform has lifted on the far side.
The distance it has moved the weight will be able to be used to assess the weight of the hive on top.
Oh I've just noticed that I didn't label the hinge (it's that blue blob near the counter weight. It's this hinge that allows the platform to tip when the counterweight is moved away from it.