Propagating yeast from commercial beers

I would like to propagate the yeast from two bottles of Bell's Oberon. I understand the process to alcohol swab and flame the bottle opening, swirl and decant, but how much of a starter (using DME) should I make (volume)?

500ml...more....less? 1.030-1.040 range?

I did this with one 22 oz bottle of DFH Squall. I made up a batch of about 1.04-1.05 wort and added a small amount of the wort straight to the bottle. Waited a few days, added some more, waited a few days, put into fridge, decanted off liquid and then pitched another small starter the night before brewing. After pitching to fermentor, fermentation started by morning. Good luck.

I just did this with a whole sixer's worth of Bell's Pale Ale dregs. I used about 470ml of water to 1/2 cup of DME, and after a few days (in a flask on a stirplate, with foil), added a second batch of the same. I then chilled that for a couple of days, decanted the old liquid, and did the same thing a second time the next week. I did so because after chilling the starter the layer of yeast didn't seem quite enough. Good Luck!