prime rib - without bones - cooking time changes

Hi,

i've done a few prime rib's on my Egg in the past 4 years:

http://www.hakalugi.net/gallery/Foodstuff/christmas_prime_rib_2006

always the same though:

begin at 450, then drop to 275-300, basically 'oven style' with a smoke ring and flavor, never before a true slow and low, like some do.

so i'd like to try the slow and low, but there's a twist:

i always buy bone-in prime rib/standing rib roast, and cut-and-cradle it myself, so the bones are a 'roasting rack' i put over my indirect contraption. not this time, my place was out of choice bone-in, only prime bone-in or choice boneless. their choice is always good and at $19/pound for the prime...

so, the "it'll cook faster" addage since it won't have the bones on it is further compliated b/c i'm thinking of a 200 degree cook this time - how far off will this throw the min/pound ratios out there?

(it's 7.75# boneless, 9-10" long and 8-9 wide.)


also, we usually just food processor: garlic, fresh rosemary, salt, pepper and olive oil, any other favorite rubs i should try in honor of this slow/low cook?

thanks!