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Conductivity meters estimate TDS by multiplying by a factor of 0.55-0.75 that depends on the ionic composition of the water. The definitive method is gravimetric: evaporate a known volume and weigh the residue. Comparing the two gives the site-specific factor, which should be established rather than assumed.
TDS from conductivity
TDS (mg/L) = conductivity (microS/cm) x conversion factor
Gravimetric TDS
TDS = residue (g) x 1,000,000 / sample volume (mL)
Different ions carry current differently. Water dominated by sodium chloride has a higher factor than water dominated by bicarbonate, so a single factor cannot serve all sources.
No health-based guideline value, but palatability declines above about 600 mg/L and most standards set an aesthetic limit around 500-1000 mg/L.