1/4" seems very thin for a 24" magnet rotor. It's more than adequate magnetically but I am not sure you will be able get a close air gap without it warping, there is a lot of force on it.
I agree with Dan that your stator thickness will be limited by mechanical considerations in this case.
For a dual rotor the optimum air gap is about 1.5 times magnet thickness but you will probably not have a stator that is rigid enough at 3/8". For a single rotor it works out about 2/3 magnet thickness but the laminated backing can support the stator in this case.
With Neo the demagnetising line is linear and you can alter the air gap quite a lot and still work well. If you go for a very small gap you tend to loose out as the physical air gaps take up a lot of your winding area and the loss of winding space tends to more than offset the gain in flux.
Your thin magnets are better suited to a single rotor at this diameter.
Flux