I agree with moneyshot, and it is also my strong opinion that we should all have the little 2 ounce bottle of GH liquid pH indicator handy at all times to verify that indeed our pH calibration is correct. This is similar to the 'outside check' concept in analytical chemistry, where you check the accuracy of your calibration on a solution of known pH that is NOT one of the calibration standards- but in this case, you're using a different TOOL to check the SAME solution. Well, your res anyway.

If you own a pH meter you also need to have a set of calibration buffers, or you're wandering around in the dark.

Also, if you are using distilled water you have to use calmag.