I think that's a good start or a slight bit more to be safe ... Worst case, you could always go diagonal if it doesn't shink as much as you expected
Sledneck is an expert on this technique though.. Hopefully he'll see this and chime in... He uses a Stanley FatMax Extreme with Blade Armor coating for an 12' standout from across the trailer to meaure his pre-trimmed brisket. I wouldn't believe it but I saw it with my own eyes.
Lonestar 24" Vertical offset w/ Insulated Firebox & Performer Platinum
Former owner: Jambo Backyard, Klose BYC, Med. Spicewine, Pitts & Spitts, XL & (2) Med. BGE's, 22" WSM & (2) 18" WSM's, 18" & 22" Weber Kettles