I only just got my Uno a week or so ago, so I'm still somewhat new to this. I ordered a kit from ebay with a 5v 28BYJ48 stepper included. It didn't come with a data sheet, but it seemed similar to the others online.
I tested it as accurately as I could with no tools and some arduino codes (mostly the oneRevolution example) and for it to do a full revolution with the standard stepper.h library, I have to use "2046" as my stepsPerRevolution value. I assume the way I have it wired means it's doing full steps vs half steps or something, but even doubling that would result in 4092 instead of 4096. Additionally, when I use setSpeed(2), it seems to take about 28 seconds rather than 30 to complete a full circle.
Any help with this would be fantastic, though I know it's not the point of this thread. But my motor does have an unusual number of steps to go full circle. I haven't tested to see if it's exactly one circle, nor have I opened it to check the gear ratios, but hopefully it helps.