TBH I haven't ever found it useful, but then it depends entirely on what you're buying.
It ranges from silly (as in 80%) on really obscure filters that presumably no-one buys to barely noticeable, to lots of things that it can't be used on e.g. the decent bulbs.
EDIT: Sorry didn't notice you specified tools.
OT: Halfords ones seem overpriced for what they are? Especially considering the length of the handles aren't that generous for the cost (Yes, obviously you can just push hard in some cases, but for wheels why not make it as easy as possible) I have a big 1/2" drive silverline one at home which seems fine, only used it for a couple of jobs. Use Halfords ones every day at work, bit nicer handle, nice gauge etc. Not a clue about calibration.