Head for a light colour that should be neutral grey - around the 80-90 mark on that toolbar. The numbers will not be equal. All you're looking for is the fact that it's light grey - and you don't really need numbers to tell you that.
TBH, I usually turn the loupe off (in the toolbar) - it's quite distracting!
I use the numbers because when White Balancing tons and tons of pictures, your eyes start to play tricks on you. Neutral gray starts to morph into other colors. Nevermind if you're monitor is not calibrated. At least if you're monitor is not calibrated, you can still go by the numbers to ensure you're attaining the right neutral gray.
Eye'ng it just don't cut it.