Hi
You can still have a formula in Column C and have it as your input cell when someone makes an entry in Column B.
Make sure that when the setting for moving to the next cell when "Enter" is pressed is set to "Right" and not "Down". That way when the use enters "Mileage" in Cell B2 the message immediately appears in Cell C2 and it will be the active (input) cell. You can have as many as 7 If statements with differing messages if you need them.
Hope this helps.
Good luck.