Most TDS meters really just measure conductivity, the amount of ions (charged particles) in solution. This isn't really TDS at all, but based on some generic assumptions, the conductivity number is converted into something approximating TDS, but it is all based entirely on measuring conductivity. Presumably your GH booster contains calcium and magnesium, which should raise both the GH and conductivity directly and linearly. If all else stays the same, you can add GH booster until you get the desired GH (using a non-expired GH kit), measure the conductivity (with your TDS meter) at that GH, and just use conductivity as your metric from there on out. If the concentration of the GH booster is constant from bottle to bottle, you shouldn't even have to do that; the amount of GH booster you have to add to your RO water to achieve a given GH should be about the same each time. Of course, as your RO membrane ages, the conductivity of your product water will gradually rise (possibly from Ca and Mg, but possibly also from Na, Fe, etc. which do not contribute to GH), but you should be monitoring that (via conductivity) independently anyway.