You could set that field to auto-enter whichever you want to be the default. Then it would set the radio buttons to that one.
You just need to remember that if you ever change the value list you also need to change the auto-enter settings.
And with an auto-entered cacluation (rather than a value), you can even set it up to enter the first (or any other position in the list) value of the value list without specifically identifying the value. That option might serve to set this up so that subsequent edits of your value list automatically update the value auto-entered as the default.