The precise calculation length is important. Because if 260/262 is already rounded up then going up another 2mm is going to be more of a problem than if the 260/262 length is rounded down.
There is a bit of flexibility as you can have the spoke threads extending a couple of mm above the top of the nipple, although you need to think about how much thread remains engaged inside the nips, it depends on how much of the spoke is threaded etc. NOTE: This is assuming you have non crap spokes where the thread is rolled in rather than cut in so the nip won’t bottom out at the bottom of the thread.
Another thing you could do is use nipple washers, not a bad idea on stans rims anyway as they tend to be lightweight and without eyelets.
Something else to consider is if the spokes are worth the effort, are they nice double butted Sapim/DT or something of lower quality?
EDIT: poosticks, I’m mostly repeating what others said – I was on the phone for half an hour before I got to press send.