Due to my hand problems, my knitting gauge can be (and usually is) irregular. If I'm feeling good, it's loose, if I'm in pain, it's tight, if I'm on medication, it looks like yarn vomit... you know. So when I knit gauge swatches, I get a little more elaborate than usual when I measure them. I'm gonna tell how, so you can be a math geek, too!
See all those pins sticking out all over? That's because I measure the gauge in at least three places (on this one, I measured four). And I measure over three inches because, well, in this case the swatch wasn't really big enough to do it over four inches. And it's a fine gauge, so you don't really NEED to do it over four. (The bigger the stitches, the bigger swatch you need, and the more inches you need to measure over, for accuracy.)
I measured in four places, and got three different gauges (this is normal for me): 17 stitches, 17.25 stitches, 17 stitches, and 16.5 stitches. Averaged (add them all together and divide by four), that comes to 16.93 stitches per inch. Yes, I usually go out to two decimal places. It helps accuracy and I'm using a calculator anyway so it's not like that makes it so much harder.
So, 16.93 stitches over three inches comes to 5.65 (I rounded) stitches per inch.
Yes, I use gauges like that when I'm knitting sweaters for myself. A sweater that's 22 stitches across the front would be 124 stitches, whereas if I'd rounded down to 5.5 stitches per inch, I'd be casting on 121 stitches. That's about a half inch difference. Add that to the back, that's an entire inch different in the circumference of the sweater. That can REALLY affect fit.
Oh, and if I'm working on something REALLY fidgety, I will measure the gauge before and after washing, figure out what percentage it changes by, and then use that to monitor my progress while knitting. For instance, if my washed swatch is 5% smaller than the washed one, and my sweater measures 5% smaller than I want it to be while I'm knitting it, odds are good that it will be perfect once it IS washed.
Nitpicking? Yeah. Detail-oriented? Sure. But you know what? My sweaters fit when I'm done knitting them. So there. Nyeah.