I'm a new member here, glad to join the group. Just thought I could help with this question.
The magnetic flux across the air gap depends on the reluctance of the complete magnetic circuit.
the formula is:
flux = MMF / circuit_reluctance
MMF is fixed according to the magnet grade and thickness. The circuit reluctance depends mainly on how far the magnetic flux has to travel through low permeability material (air, copper and magnet). Interestingly, the permeability of a Neodymium permanent magnet is similar to air.
So the relationship would be flux = k/g
where "k" is a constant and "g" is the total gap between the iron rotor plates. (not the gap between the magnets).
So for instance, if the magnet on each side is 10mm thick, and the gap between the magnets is initially 10mm, then the total "air" gap is 30mm. If you increase the gap between the magents to 20mm, then the total "air" gap becomes 40mm. The amount of magnetic flux in the this new configuration will be about 30/40 = 75% compared to the initial case.
Therefore the induced voltage (at any given RPM) will be about 75%. (assuming you don't change the number of turns in the stator)
hope this helps,