when to do med exam for gc?

My LC was just approved and my attorney is going to file my I140/485 currently. Is there anybody who can tell me when to do med exam for gc---before filing I140/485 or waiting until USCIS informs us to do it? Also, how long is med exam result good for? Would you please provide a link of designated physicans for GC med exam? Your experiences and advices are highly appreciated. Thank you so much!!!