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TDS is 85% of Brix, use this extraction chart as a guideline as to what the tds "should" be for your coffee:water ratio. Note how the 19:1 ratio has the biggest window of "acceptable" tds values. Please don't let this dogma blind you ...
<Hand raised>
So, brewed a vacpot of Dominican Corazon de Oro. 68 grams to 42 oz water. TDS came back 122 at room temp with the refractometer at 1.20. When I am looking at Bunn's (SCAA) chart, I am not sure what I am reading.
Is the chart measuring refraction or TDS?
...and yes, it was a very good cup (primary measurement).
B|Java
The chart is showing TDS, you get your water:coffee ratio and plot it against TDS solving for extraction rate.
So (42oz * 30g/oz)/68g gives you a nice 18.5 ratio, which is pretty much the middle band. The middle band gives the most area in the "ideal" box. I tend to dose at a lower ratio, (more coffee) I use the higher bands and have a narrower "ideal" window.
TDS measuring 1220 would put you right in middle of the box. The refractometer reads in Brix, which correlates to TDS/850. If I multiply 1.20 * 850 I get 1020, which puts you lower left of ideal.
The Bunn chart recommends you will get an even better cup if you grind a little finer, or brew a little longer. Although it is interesting to see what their recommended cup tastes like, the meter can also indicate how consistent your brew method is in terms of strength, and your general preferences vs their "research".
Also interesting to test local coffee shops, compare against same technique/coffee/dose on a friends grinder, etc.
One thing I forgot to mention:
The refractometer can be calibrated by using distilled water. The line should go to zero, the scale is adjustable with the set screw on top. The unit will read strangely if the prism and the fluid measured are at different temperatures, and will always read weak if the coffee is warm at all.