there are 2 ways to calculate EC. If dividing by .5 doesn't match up with ppm, divide by .7 and see if that corrects it.
if yo have a nice RO system, you can mix up some measured amount of nutes and calculate what the PPM should be from the guaranteed analysis labels. then, compare. A really good RO system wil be near 0 ppm, if not mistaken? if not, that's what you'd need, then mix and measure.. start with a known.. compare measurement. easy as pie.
look into re-callibrating, if that's something necessary for those pens. that'll also verify integirty.