contest butt timing

I'm doing a contest timing on butts/ribs this weekend on a friends 18 wsm that I'm going to borrow for a contest in a couple of months. This smoker will be the butt/rib cooker for me & I think I've got a pretty good handle on rib timing but he's not done a butt on it so doesn't have any timing tips to give me. If I want the butt to come off at 4pm what time will I need to start/foil (approx.) w/ cook done at 250-275 range? The cooking topics here shows 12+ hrs cook for all of them but no mention of foil so assume it'd be much quicker if foil was done. I assume the butt will be in the 8-10lb range but haven't got it yet. I've done many butts but did them on my BDS so I know times there but that is my brisket/chicken cooker so no room on it for these. Any help/advice appreciated

I am NOT a competition cook. However, the last several butts I have smoked on my 18.5" WSM as well as my 14.5" have been done at 275 grate temperature (Maverick 732) and have all been 8.0-9.0 lb butts. They have taken about one hour+ a pound. That is, 80-9.0 hrs. for an eight pound bone in Boston butt. In every case, I have foiled after the bark is set. They were tender at anywhere from 195-205 degrees internal temperature and all were pulled with excellent quality meat (moist, tender, etc.).

I put my meat on when the smoker is approaching 200 degrees and the meat has set on the counter (out of the refrigerator) with rub on it for about an hour.

FWIW
Keep on smokin',
Dale53