Relax. Low cost carriers actively try to catch people out with these things, legacy carriers generally don't. Although I am told it helps if (like me) you are tall (because the bag looks smaller in comparison!)...

1cm here or there is very, very unlikely to be picked up.

I have never, ever, ever been asked to use a sizing gauge (although I have never flown a LoCo) - even in India where RULES ARE RULES (LOL) Since I just passed two million Oneworld FF miles, that goes to show how rare it is on legacy carriers. The one and only time my hand luggage was checked for any sort of parameters was by airport staff in Sydney who insisted on weighing it (...?!)

So my advice is not to get worked up about it, and if you are challenged, plead ignorance and say "but the (wo)man in the shop told me it was regulation size"...