I just took a trip backwards in time to when we first moved into this house about 14 years ago. It had a single dinosaur Bell & Gossett Series 100 circulator back then. I pulled up the pump curve for the Series 100 and it delivered zero GPM at just a tick above 8 ft. of head, and (perhaps) 5 GPM at exactly 8 ft. of friction head. At 7.5 ft. of head it delivered 12.5 GPM (which is sufficient to deliver all of the output BTU's I can ever hope to muster from the S130 Coal Gun, plus some). And at 7 ft. of head it delivered 15 GPM (which for me would certainly be entering the realm of "velocity noise" territory).
By the measure of the Series 100 circulator (since 5 GPM feeding 4 zones would just barely cut the muster for BTU delivery into my combined zones) my systems maximum friction head must be set at 8 ft. or lower.
My latest calculation (using the B&G rule of thumb method) indicates not more than 7.8 ft. of head. The B&G rule of thumb for 3/4" piping (plus the rest of the system including the boiler and all elbows, valves, and fittings ) says to first measure the total length that water travels through your longest zone, including the boiler itself. Then multiply this times a cheat factor of 1.5. Then multiply the result by 0.04 and this yields the maximum friction head for most closed loop systems. But to make things even more simple you can multiply the overall length traversed by 0.06 to get the same result.
Doing this for my single longest zone I get.
~130 feet x 0.06 = 7.8 ft. of friction head.
So based on the circulator I found installed when we moved in I can't have more than 8 ft. of head, and based on the B&G rule of thumb I can't have more than 7.8 ft. of head, I find these two measures to be in close agreement as to the head "ceiling". And 7 ft. of head establishes the head "floor".
In addition I find it comforting to learn that for a multiple zone system the total friction head to be overcome by a single circulator that supplies all zones (such as I have) will never exceed the friction head of the single longest zone loop. This is because friction (resistance to flow) decreases as additional zones open to flow.
At least now I know that my systems friction head demand must be no higher than 8 ft., and (by an admittedly conservative rule of thumb) should be no more than 7.8 ft (and in reality is likely to be a bit below 7.8 ft.), and can likely be no lower than 7 feet of head.
My Taco 00R 3-Speed circulator when set at low speed delivers about 6.2 GPM into 8 ft. of friction head and 7.2 GPM into 7 ft. of friction head. I'm probably right between these two extremes as to its GPM's of flow. Call it 6.7 GPM. That is sufficient to carry 67,000 BTU's to the emitters at a Delta-T of 20 degrees. It looks like I can leave it on low speed.
One more confirmation: I've been monitoring the supply and return temps, and noticing that when my longest zone is calling for heat my circulator runs for 5 minutes before supply and return temperatures are beginning to settle down to a consistent Delta-T. It also takes 5 minutes for the boilers outlet (supply) temperature to make its first downward tick, indicating that the returning colder water has finally reached the boiler outlet after having made one complete cycle around and through the boiler. My longest zone loop plus my boiler have a combined capacity of about 33 gallons of water.
33 gallons / 6.7 GPM of flow = 4.93 minutes.