Honestly I wouldn't assume one is a gold standard. Fresh reagents are of course always a good idea.
As long as they are consistently wrong you're fine. I used to keep my dkh at like 11.5, but decided that if my tests were a dkh point off it would put me closer to the high end than I prefer. Now I shoot for 9. 10 or 8 are acceptable if my Hanna checker is a point out of calibration or something -- as long as it's the same direction each time.
Stable one point off is better then fluctuating a ton but more accurate.