You can absolutely start now!
The ultimate aim of the jam is simply to get more gender bender games out into the world, so we aren't strict about the exact percentage of work done during the official timeline. The rule is mostly there to prevent people from submitting old games they made years ago just for promotion, since that doesn't really help create new content.
Don't stress too much about hitting an exact 50% mark. Feel free to jump in and start creating whenever you're ready!