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An easy solution (sorry) is to make a graph using control batches of different percentage concentrations and then in future you can compare refractometer reading to concentration.

If you can find spares easily this may be the way to go; if not then keep an eye open on Ebay for a few months, i purchased a Dronsfield Eagle Mk4 for £250 about 3 years ago, it still needs a rebuild...

Have a look at the Denford forum of old machines or Boxford cnc machines. They both have small old machines which have ballscrews. Electronics conversions are relatively cheap.

It will be much easier to buy an older machine with ball screws fitted which has obsolete electronics and fit new electronics.