Orsat Oxygen Testing Procedure

I have read all the posts on the site regarding oxygen testing using an Orsat apparatus and the general consensus seems to be that any mechanical or instrumental analysis is better. However the actual Orsat apparatus is what I have to deal with. I need to test compressed oxygen per the USP monograph for 99% oxygen. I have the apparatus and it seems to be working as I expect for the most part but I just can't quite get the accuracy that I need. I am coming up 0.5-1% low on my analysis of several gases with a certified O2 concentration. From experience, is this to be expected or can accurate results be gained using this apparatus. Also, the method that I have come up with is based on bits and pieces of information that I have found online and lots of trial and error. Does anyone know of a good set of instructions that I could find to help me out. Thanks in advance.