I've now done this like so:
[libraryCell.myView bind:@"amount"
toObject:libraryCell
withKeyPath:@"objectValue.amount"
options:nil];
[libraryCell.myView bind:@"max"
toObject:_librariesController
withKeyPath:@"arrangedObjects.@max.amount"
options:nil];
There are thus two properties in myView (amount and max) and when either of them change I do the calculation and update the display accordingly.