System efficiency and run time

KBP&H_3

Does anyone know how much you might be able to save ( or gain) by adding a 50 gallon buffer tank to a regular system, -ie, 120,000 btu boiler, 4 zones ( 40,000; 20,000; 13,000; 10,000 btu) Taco 008VDT circ, standard outdoor reset,  average run time now is 2-4 minutes,,,, ??? yearly usage last year for heating propane = 1100 gallons,,, 6100 degree days, my propane co used to print the K and other info on the ticket, now they say it's confidential??