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 <kbd>b</kbd><kbd>c</kbd> 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.*