I have no idea.

There's no rules when it comes to lighting, but if you want to maximize the area of the umbrella, the flash should be as far away from it as possible without spilling out of the sides. Imagine the flash pressed up against the fabric of the umbrella--how could the light spread out and soften that way?

Very good. Thanks!

No problem! Have fun and keep experimenting. Try the flash a bit closer for a more intense hot spot, or aim the umbrella out in front of your subject (called feathering) to hit them with just the edge of the light.

....also depends on the zoom setting of your flash.

With flash zoom head set to 24mm coverage, in order to fill the umbrella the flash needs to be as far out along the shaft to be the same as the diameter of the umbrella

So set 24mm coverage, put it 24" out from the curve of the umbrella, to fill a 24" diameter umbrella fully.

To help prevent hot spots in the center the mast should be extended as far out as possible, chocking up on it prevents the light from spreading out evenly