I would suggest using a kitchen scale to weight the food (minus the container weight) and then add in the 3% of salt (or whatever percent you want). Using the scale made all the difference for me with getting the consistency and flavor. good luck!

Two of my most used and most useful kitchen tools are a Thermapen and a digital scale. Just being able to measure accurately makes all of the difference in cooking just about anything. A cup of flour or a teaspoon of salt can weigh very differently depending on how tightly packed or how fine the grain is, better to be exact and not risk screwing everything up.

I love Thermapens! I've been looking for an equally well built kitchen scale. Have any recommendations?

The 0.1g-precision Bluetooth scales used for coffee are decent. They all seem to share essentially the same firmware, which suggests that they come from the same ODM, and they’re decent — mostly work, only a little flakey.

If it's hot in the kitchen (I ferment on the counter so I don't forget about the ferments), I ere on the side of caution and use quite a bit of salt. After a few days I taste the brine. It should be starting to sour. If it's clearly too salty, I just add a bit of fresh water to the crock, taste again, and repeat.

I taste it every day and refrigerate the ferment once it's at the right sourness.

Using volume measurement for salt is a bit problematic, because different brand of salt come with different sizes. A teaspoon of finer salt might be twice as heavy as a teaspoon of coarser salt. It's not always easy to find out that ratio between your salt and the one used by the recipe author.

I wonder if the gasses could be sampled and monitored, or maybe some spectral analysis of the fluids, to find if undesirable biological activity were happening?

I use litmus paper to test acidity of the brine. Usually ends up around 3.0-3.5 ph.

Also monitor smell, bubble formation (good) and watch for bad colors (white, black, green mold on top), browning of ferment, especially near the top (bad)

Salty fermentation products are good for stir fries or soups where a little goes a long way, and you can dilute with other ingredients.