I got the tri-tip from Stater Brothers. There were two to the package and it was on special for $2.49 a pound. Not as good a deal as yours tri-tip but I couldn't pass it up.
I checked the rack temp with my Redi Check thermometer and after the lid was close for awhile and things warmed up, there was only about 10 or 15 degrees difference between the rack temp and the pit thermometer's readings.
I modified my charcoal basket some so the size is equal to 1/3 the width of the grill and same depth as the charcoal screen is sits on. I basically have three different zones for direct cooking - far left, center and far right. It seems to work pretty good this way. I've been cooking with it in the center position so I end up with a non-direct zone to each side. The zone on the left is about 10 degrees cooler than the zone on the far right. I think that's because the wind was blowing pretty hard from that direction.