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I was making them for the guy at SBC (at that time I didn't have a nipple holder/driver thing, for deeper rims I was just using a bit of spoke threaded into the top of each nipple to insert them) and he reported that the first one I made him the nipples would push down between the slats when he tried to pick them up with his holder. I think I then made one that was a touch too tight and had the be shaken for ages to get the nipples sorted and then we eventually got a good spacing.
I'd have to look at the file to see what it ended up at.
I think I just went for 4mm which seems to work pretty well. There are a couple of tighter slots as the cheap crafting balsa ply isn’t that flat out of the packet so I don’t think fractions of a mm on the slat spacing will make a difference to the functionality.