The old school rule of thumb states that your bandsaw blade thickness should be 1/1000th of your mill wheel diameter. For example, if your mill has 42″ diameter wheels, your bandsaw blade should be 42″/1000= 0.042″ or 19 BWG. This rule was used in earlier days when saw cracking was a much bigger problem.
Today, modern mills have improved tolerances and design, bandsaw steel has better tolerances and composition, and improvements in sawfiling technology have made it more practical to go one gauge heavier than the old rule of thumb. So, in the above example of a mill with 42″ diameter wheels, you would use the formula and then move up one gauge. Bringing your gauge to 18 BWG or 0.049″.