I just purchased a digital water tester that measures pH, temp, and EC/TDS. Does anyone know how TDS is related to KH? Is there a formula or ratio to convert one to the other?

TDS (total dissolved solids) measures the gH of your water in parts per million. It doesn't measure your kH. Although if your TDS reading is 150 or higher, chances are good that your kH is high enough to be stable. The PH reading is a much better indicator of the kH of your water. The higher your PH reading is... the higher your kH will be.