With the setup above, splitting up threads across injectors gets confused - in practice I found it trying to give me extra users as it couldn't split 1 user from the initialise thread across 3 injectors for example.
Also, the way I intend to work with these jmeter scripts is to test them on the target injectors to see how many users I can run per injector. This is highly dependent on the script itself - how many objects it uses etc. So once I have established the limits of users per injector, I can then leave that and increase users by increasing injectors. I feel happier myself having this more direct control on users per injector.
So I've effectively commented out the threads section of the controller. This can easily be reset if it fits your needs: