Filling the air gap only applies to black powder loads, not smokeless (which would be extremely dangerous!). A good reduced black powder load is 50gr of FFg (or Pyrodex C) and then fill the rest of the case with filler. Cornmeal compacts too much and would act like an obstruction, so I use Cream of Wheat. Fill the case to about 1/2" from the mouth, depending on how long your bullet is. I then put a 45 cal wonder wad and then the bullet. The seated bullet should just slightly compact the load. If the bullet starts to come back out before crimping, you're over compacting, so dump some of the filler.