Before you use that hygrometer, you want to calibrate it, so you know if it is reading correctly (really important because while my Springfield is spot on, I've heard of them being as much as 15% off). To calibrate, take a mug and put in 1/2 cup salt and 1/4 water, then mix them up. The consistency should be like wet sand. Then put the mug in a ziploc bag. Place your hygrometer in the ziploc bag with it (but not in the mug). Seal the bag and leave it to sit for 24 hours. Read the hygrometer at that point. If it is reading accurately it will say 75%. If it is anything but 75%, that is how much you'll need to mentally adjust by every time you read it. For example, if at the end of 24 hours it is reading 70%, you know it reads 5% low, and can make the adjustments during incubation accordingly. Note that most directions say to only leave it in the bag with the salt for 12 hours. When I calibrated mine, after 12 hours it was reading 72-73% but after 24 hours it was up to 75% and held THAT for a full 24 hours before starting to drop. So I will always leave it for 24 hours now before deciding it is "done".
I know you don't want to wait another 24 hours to set your eggs. Go ahead and start the calibration. Set your eggs when you planned to and then add the hygrometer when its done calibrating. If you don't know the exact humidity for the first few hours of incubation its probably not going to be a big issue.