Assassin temps

I just got a new to me assassin 17 gf. I will do my first ever cook on a gravity fed tomorrow. I like to hold my briskets for a few hours after cooking. What is a reasonable low temp that the assassin will hold steady at? Ideally I want 140 degrees. How does it respond if you are smoking at 250 and decrease the pit to 140? How long would it take to drop to 145? I am using a pit viper fan and a FireBoard controller. Thanks in advance