The amount of replate medium bears a direct relation to the nature of the flask you intend to put it in. I've heard that it is a good idea for the first replate to be on media about 1"-1.5" deep.
This is the stage you will most likely find the most frustrating. I think its very difficult to get tiny seedlings or corms from one flask to another without contamination, but that may just be me...
I tried an expariment with this situation, Instead of preparing new replate flasks, I simply poked a hole in the cap of the mother flask. And left another mother flask completely unvented. The unvented flask suffered slight damages with overpopulation, but survival of the fittest sorted that out for me. Both flasks were never replated, one year later, deflasked and compotted. the seedlings from the vented flasks were a tiny bit taller, but to save all effort and woe, well worth the sacrifice.
Also, It is best to replate when the corms reach a size that you can easily manipulate with your chosen replate tool...i.e If your using a spoon, you want bigger corms first, a little bent wire (my fave) you could probably do it any time. I like to wait untill the central growth of the plantlet starts to pop out from the side of the protocorm, that way you know the seed has found the neccessary fungi to make the "corm to plantlet" transition without damp off.