Thanks, Dean, that makes a lot of sense. I'm actually using Castrol Hysol XF. When I look at the datasheet, there's an entry called "refractometer factor" that I hadn't noticed before. For this stuff, the factor is 1.1, so hardly worth bothering adjusting for.

In my case, I'm forever having to top it up with water to make up for evaporative losses - I guess that's the price you pay for a heated workshop. I just top it up to the level and periodically check the concentration.
https://www.thelubricationstore.com/...FDataSheet.pdf