Have settled on the Biasi boiler but have a sizing question. If I measure the total number of feet of baseboard (measuring fin to fin) which equals 115 feet X 600 btu gives a MBH of 69. This is what my boiler guy said to do! The MBH Heating capacity of the B-3 model is 67. If I go up to the B-4 the MBH is 87 to 110. (87 per the brochure, 110 at the QHT website. Is there a fudge factor? If my math is correct, I have to reduce my total fin feet by 3 feet to match the MBH. Otherwise I have to go to the much larger B-4 which doesn't seem right. Confused! Bill