Changed a faulty gas valve on a maxin 24e, set up the burner pressures as per MI's with digital monometer. After this i stuck my fga in test point. I havent done my cpa1 yet but used it out of interetest and to try it out. When it started picking up readings the co level went it the thousands. I put it off and checked everything i'd done. Then i checked the boiler, flue etc. The boiler looked like it hasnt been serviced for a while, so i cleaned the burner and heat exchanger. After that i tried it again and the co level was down to 252 ppm and the ratio was 0.032%. in reading new boiler MI's this is a bit off. Is this ok or does the boiler require further cleaning??