The 442 conversion is approximately 700 x EC in millisiemens (mS). Then there is the NaCl conversion (sodium chloride) which others say is the closest thing to a hydroponic solution. The NaCl conversion is approximately 500 x EC in millisiemens (mS).
General Hydroponics Calculations:
To obtain an approximate sodium chloride TDS value, multiply the EC reading (in milliSiemens/cm) by 1000 and divide by 2.
To get an EC value, multiply the ppm reading by 2 and divide by 1000.
Thus, if your EC is 1:
1*1000/2= 500 ppm.
And if your ppm is 500:
500*2/1000= 1 EC
Scientific Hydroponics: http://scienceinhydroponics.com/2013/02/the-first-free-hydroponic-nutrient-calculator-program-o.html
Greentrees Hydroponics: https://www.hydroponics.net/learn/debate_over_ec_and_tds.asp
General Hydroponics: http://generalhydroponics.com/site/index.php/resources/faqs/tds_ec_ppm/