Stormax handheld reader can have new moisture charts updated for it for various crops. I did that on my reader (requires a USB cable and software).
All moisture cables work the same way. They simply read relative air humidity. Then they use a chart to lookup that up with temperature to get an estimate of the grain's moisture content. No matter what system, it's just an estimate, and the accuracy is always going to better at certain values than others. Partly these charts are generated by mathematical formulas, and partly by empiricle measurement. No manufacturer can guarantee they've made measurements for every combination of temperature and humidity, and they can't account for every little difference in your grain varieties.
The stormax cables are accurate enough if you're within a couple of percent of dry. If you're dealing with really dry grain or realy wet grain, the charts just aren't that accurate. Just the nature of measuring moisture this way, regardless of what system you go with.
Theoretically if the units allowed altering the curves with manual data entry you could tell the reader, hey this is 20% grain, and it would adjust things. But the Opi handheld reader is not a very powerful little unit. Just an 8-bit microprocessor with limited computational skills.