That's a revelation to me--that Toro ever produced a BBC that was Tecumseh equipped. I spent 7 years in a Toro shop from the late 90's to the early 2000's and never saw one in the shop. Rather surprised that a Tecumseh of that age is still running--you must have given it exceptional care.
It's just me but I would set it at 3000 under load(blade engaged) and be done with it. On the other hand, if you're still on the unaltered original engine, leave it as it has been.