said, you really need the 342 ppm calibration solution to correctly calibrate. If you want to double-check that the 342 solution is, in fact, calibrating it correctly, you can do this:
1) Get some, e.g.; KHCO3 (or any fert salt).
2) Use the Rotala Butterfly | Planted Aquarium Nutrient Dosing Calculator
calculator and pick a target dKH for one gallon of distilled or RO water. Maybe 8 dKH (easier to measure dKH at higher numbers).
3) For this example, it returns 1.08 grams in 1 gal of pure water = 174 ppm of HCO3 and 111 ppm of K.
4) Add the 1.08 grams to the pure water (assumes your scale is calibrated, which is another story) and make sure it's dissolved well.
5) Sum the ppm numbers, which will return 285 ppm.
6) Stick your TDS pen in the gallon of water and you should get something very close to 204 ppm (if using a 0.5 conversion factor TDS meter - most US TDS meters) or 285 ppm (if using a 0.7 conversion factor TDS meter). Also, take a KH reading to see if you're close to 8 dKH just as triple-check.
Any of the above variables can be substituted with different salts and concentrations to double-double-double-double and double-check to your hearts desire.