Good question.
Doc says he doesn't get much over 18 and his are a bit of a benchmark, so ...
Mine typically read 10-12 when I first start testing them in late veg. From there, they'll drift upward to 13+ before harvest.
Doc also figures that his plants are safe at 12+.
Technique matters and I've explained mine before, but I'll do it again.
Doc takes his tests from the tops. Most of us take ours from average middle leaves - like your typical 5 blade leaf in the middle of the canopy - there's always one in there that looks fine, but is in the way. Since the petiole will read much lower, I'll snap off any "extra" - y'know, some are really long - leaving .75 inch or so, y'know, average.
I twist the leaf and roll it up, keeping the petiole inside the roll, then flatten without releasing any juice. I use vicegrips and a couple coin-sized metal discs, and just firmly press it flat with a few squeezes. Then I pull the discs apart, roll the flattened leaf back into a ball and squeeze the discs over the prism. You can just tap/touch the edge of the crush to the prism and get enough for a reading. I like to take 2 in a row, so I'll open it back up, roll it back into a ball, clean off the prism and crush a second drop. The more tests I've done, the closer the 2 readings have gotten, usually the same now. You should also always take your readings under the same conditions and at the same time in the light cycle. Dry or necrotic leaves will read higher and freshly watered plants will read lower, for instance. Sap sugar varies through the growing day. But once we get a better handle on the readings, I've read from vinyard usage that you can read a response from a treatment later that same day. We could really get some good data on how the plant responds.
I'm happy to hear you're going to start testing brix! We're at the very first stages of using it with cannabis, and it'll be good to have another grower taking readings.
[Edit] And thanks for the support~!