TDS meters are simply cheap electrical conductivity (EC) meters with a built-in conversion factor. The true conversion factor depends on the ions dissolved, so the conversion factor should change with each chemical composition tested. I recommend against relying on the number as "gospel", unless it's been calibrated, but it's still usable:
Many orchid growers use a measurement of "X teaspoons per gallon". Generally, powdered fertilizers are in the neighborhood of 1.5g/teaspoon, which means an actual contribution of about 400 ppm TDS per teaspoon used. Yeah, chemically-bound water in the hydrates used in the fertilizer formulation will reduce that a bit, but it seems like a thumbnail guide in that range is OK.
Let's say you're using the GreenCare MSU formula for well water and want 125 ppm nitrogen. That would require about 1/2 teaspoon per gallon, so you should see about 200 ppm added to your baseline, water-only number.