In practice, TDS and salt at such higher levels will be very close -- usually the TDS is around 200 ppm higher than the salt level, but this depends a lot on how TDS is measured and usually it isn't measured accurately (since you have to know the components of the water to properly calculate TDS in ppm). So just use a decent salt test and calculate the amount to add based on salt ppm (you can use The Pool Calculator for that).