Wow, you revived a six week old thread.
My refractometer is calibrated with Pinpoint 53mS calibration fluid and is spot on when comparing to my lab grade floating hydrometer.
And I don't need to test again because I am quite satisfied that my results are reasonably accurate.
If you believe in your Seachem kits, thats ok. I don't. We will never agree on this. And thats ok too.
IIRC, You tested your Seachems calcium against API's and they matched exactly.
I tested API's with Seachem and the Seachem was 70 ppm lower. And the Salifert matched the API.
If anyone can provide a logical explanation for this phenomenon please, put me out of my misery.