What are the required minimums to plan to use an airport with a precision approach on the field, whether you decide to use that approach or not?
600ft and 2sm one hour before and after your ETA at the alternate airport
What are the minimums required to plan to use an airport without a precision approach for your alternate?
800ft and 2sm one hour before and after your ETA at the alternate airport
When are you required to file an alternate?
When the forecast is 3sm and 2,000ft, or worse, one hour before and after your ETA