Forget about TDS, if it were important in keeping a sanitary and sparkling troublefree pool, we would be on the watch for it. But it isn't and we don't.
To properly test your CYA.
Go outside in natural light
Turn your back to the sun
hold the test tube at waist level
Squirt the test solution into the test tube until the black dot disappears
Most of us pour the solution from the tube back into the squirt bottle and repeat the test a few times so we can be sure our firts reading wasnt a false one.
Of Note. Its important that the water be above 70F. If it isnt, then bring the water inside and let it warm up to room temperature, then do the test.
Thats all there is to it. Trust your test kit and test correctly and it will give you the true CYA value.
Its pretty much that simple.