Time vs. Weight vs. Temp

So I'm cooking St. Louis spares for competition. I'm running the pit at 250*. Starting weight is 7.4lbs per rack untrimmed perhaps 4.5lbs after trimming them (i'm guessing here). 2.5-1.5-1 gives me what I want for a finished rib at this weight and time combination.

I can't always find these honkin ribs throughout the year. The next week I can only find 5.3 lb spares at starting weight perhaps 3.5lbs trimmed. The next week I'm down to 4.8lbs....(which I won't buy) I still need to maintain 250* house temp because I have a beeve cooking too.

Any ideas as to adjusting my times for lighter weight ribs and where?

I was thinking of peeling 10min off each of the three times for a 30 min reduction for ribs at 3.5 lbs trimmed.

Thoughts?

I've never found larger ribs to be tougher but then I'm very picky about marbling of the meat. I really like a wider rib in the box but I hate to cut to wide on a smaller rack and get the knuckle in there.