I'm sorry to say it but you're wrong about TDS being a tester. Both TDS & PPM are measurements of the relative concentration of a solution. There is also a third one EC.
TDS = total dissolved solids; PPM = parts per million; and EC = electrical conductivity.
They each have their pros and cons in determining the strength of a nutrient solution, but ppm is the most frequently published measurement from the manufacturers/distributors of the nutrient.
Unless you are using RO (reverse osmosis = a very effective filtration method that can remove even most dissolved minerals) or distilled water, which while it removes dissolved solids actually increases the concentration of highly volatile (a vapor temperature lower than the vapor point of water) chemicals such as things like ether, alcohol, organic solvents and so on< you need ot adjust your recommended PPM to account for the exisiting dissolved solid in your water source. For instance my tap water is 200 PPM, barely hard, so if the nutrients I'm usig call for a PPM of 300, then to achieve that strength my solution would measure 500 PPM -- the 200 PPM that I have in my untreated water + the 300 PPM for the correct strength of the nutrients = 500PPM for my mixed solution.
However, one problem with anything except an complete chemical analysis is that the none of the testers really tell you how much of what is dissolved in your solution. You could have almost pure calcium carbonate (limestone), almost totally iron, a lot of nitrogen substances, a lot of phosphorus, a lot of potassium or a mixture of several of the above and or some thing totally unrelated to plants nutrition and yet have the same starting PPM/TDS/EC.
So if you have a high initial PPM/TDS/EC you should have your water analysed to be sure what is in it because if you have a lot of any particular plant nutrient then you need to adjust the nutrients to accommodate for that already high concentration of the particular nutrient or you will wind up either over feeding that nutrient or locking out another essential nutrient.
If you use well or stream water, you will probably have to pay to have the analysis done, but if you are on a public water system, they should be able to pretty well tell you what is in your water because they have to have it analyzed frequently.
