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Gordon describes a mixing procedure at a local shop, mentions that the O2 meter used in the procedure is accurate to 0.1% and wonders whether partial pressure mixing is that accurate. First of all, the procedure described by Gordon is essentially the partial pressure mixing technique used here. Analysis of the resulting mix, and adjusting based on the analysis, is always done with partial pressure mixing unless the PO2 is not critical, and even then is almost always done. An example of a situation in which PO2 is not critical is if you are using a nitrox mix for safety on 10 and 20 fsw deco stops (even pure O2 would be OK here for up to 45 min, depending on O2 exposure on the bottom), and are deco-ing according to an air table. In almost any other dive, especially those in which extended bottom time due to EAD is used or in which PO2=1.6 ATAs is approached, the FO2 in the mix MUST be analyzed. Several factors limit the accuracy of partial pressure mixing. Among them are limited accuracy of pressure gauges, and the fact that filling a tank raises its temperature, so that measured pressure of gas when filling isn't the same as when it is cool. Typical procedure is to fill tanks, allow them to cool and gases to mix, analyze them, adjust the fill, cool, and reanalyze. Although the standard O2 meters in use here (e.g., the Teledyne) have a digital readout which goes to 0.1%, in fact the accuracy of most O2 meters is limited by the sensor to between 1-3%. You can verify this yourself by taking repeated measurements of the same mix - it seldom comes out exactly the same.
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