A lot of the problems folks experience with the Hanna Ca checker are due to their RODI water used to create the blank has some impurities.What do you mean by hanna lab grade rodi?
If you have a perfect RODI system it's not a problem, but it only takes the tiniest amount of impurity to result in a incorrect Ca reading.
Easiest solution is to use a certified lab grade RO/DI which you can obtain from many chemistry suppliers including Hanna ( HI70436 for example)