Makeup trips you request will be assigned to you if available, don't ask for anything you don't want...and it will build up to whatever credit value you wish. But if you ask for 39, and only 6 of what you want is available it'll build to 6. It's not like VTO where you can/will wind up for things you never requested...it's a makeup bid.
only important safety tip is that if you ask for that cool 39ch carryover trip into August, might get it but that'll simply mean only a few hours of pay in your July bid month, which could be exactly what you'd prefer some other time