Thanks guys. Lee, I'm trying to make some progress each day, even if it's just a little bit!
Julien, I really prefer this mold technique over a closed two part mold because I can avoid resin pour stubs and vent sprues. It preserves detail and makes clean up of each part easier because all I need to do is knock off some thin flash and run the bottom edges across a piece of sandpaper a few times.
I'll definitely be using this technique to cast the other half-round parts (Saturn V and Sealab) for the main engines.
The thing to watch for is not to let the resin build up at the bottom edge, along the open part of the mold. I use a piece of scrap styrene sheet to scrape any excess resin over to the side, and then wick that away with a paper towel before it cures. This keeps any excess flash paper thin and makes for easier cleanup of the part.
Talc is definitely good for helping the resin flow. You definitely don't want too much, or it can affect the quality of your casting. I dust it on lightly and then blow most of it out with an airbrush.