If you're running a Magit version newer than 2.8 (the relevant commit was on 2016-10-10), you can customize the user option magit-no-confirm-default.
If you add magit-branch-and-checkout to that list, then when you use bc you will only need to enter a new branch name, which will be set to whatever the default start point would have been, had you been prompted for one.
n.b. That default is contextual! The prompt for the new branch will indicate what it's going to use.