a #20 sieve is 850 micron, (.841mm or 841 micron) right? You're talking US sieve number, right?
(I have a hand sieve, there's some variation in the definition of US #20, in my accompanying manual it says the #20 is 0.03" or 762 micron, but most of the universal tables call the #20 (Tyler or US) something more like 841 to 850 micron). MPE Chicago's reference table says #20 is 833microns.
50% pass through is technically coarser than SCAA recommendation for cupping:http://www.scaa.org/PDF/resources/grind-for-cupping.pdf
But I question whether they actually have it right. I think 50% pass through is the right answer.
If you look at the math, 70%-75% pass through on a 840micron screen implies somewhere around 600micron average grind - something I consider fine to very fine drip (to get here on a calibrated Ditting requires a setting of 5 1/2 or so, which is quite fine). This produces a pretty deep dissolution and fairly high strengths in a cupping.
For example, if I find a grind that produces 20% extraction with a 17.4 water brew ratio (1.25% strength) on an auto drip machine (for me that's a Ditting around 8, Bodum around 4-5, LIDO around 2.0), this only produces a strength of 1.1 - 1.15% in a cupping or an immersion method of brewing. These are approx 50% retained/50% pass through a #20 sieve.
If I go finer as recommended (Ditting 5, Bodum at approximately pourover icon or "0", LIDO around 1.25, 70-75% pass through, 25%-30% retained on #20 sieve), I get overextraction in a drip (strengths closer to 1.40%, calculated extractions above 22%) and strengths closer to 1.25% with a cupping, along with elements of bitterness.
(BTW - I know the recommendations for cupping are 8.25g coffee per 150ml, but the true mass water brew ratio is more like 17.4, not 150/8.25. The water VOLUME is hot, or about 96% less dense. Additionally, the water is added to the coffee in the cup, and the coffee does take up some more room so the actual added water to bring the whole slurry to 150ml is actually even less. Cupping seems scientific and accurate at first, but there's still a huge variation in the true brew ratio, not to mention the roughly 6-10g of evaporation that occurs in the cup during the 1/2 hour cupping.)