I know this is slightly off-topic, but I have yet to master the art of provisionally casting on, be it for socks, lace shawls, etc. I manage to fudge it typically, but it just never works right for me and I get frustrated and cranky LOL
That being said, if you plan on doing a lot of toe-up socks, I *highly* recommend learning Judy's Magic Cast-On
. It's the one I use when I teach sock classes, and everyone loves it and I love the toe it creates.
Not answering your question at all, just sharing an opinion, feel free to ignore me.
Enjoy your socks!