The idea of filling the space, was to fill the air of compressed air between the coming flame front and the bottom of the bullet. It was thought that the resulting compressed air was responsible for the odd ringed chambers that occurred with must using a small square of Dacron.
The foam filling this area removed or eliminated that possibility.
When I used kapok (from an old life jacket) as a filler, I filled the space to a compressed load. I was doing this back in the 70's, when I didn't want to go out and buy Dacron for a filler like everyone else seemed to be using. Most of the writers wrote of using 1/2gr. or 1 gr. 'square'. They(might have been Lyman) even showed pictures of a sectioned .45/70 case with a miniscule fast burning powder charge, like 10gr. unique or something, then a tiny ball of Dacron holding it in the bottom of the case.

I do think the excess space should be filled & it seems maybe foam is the best filler so far. If a section or it only as long as the air space works, that's great. If the velocity variations high to low are narrower with compressed foam, then THAT is the way to go.

The best results are what drives us.