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i know they come in 1mm increments but how much leeway do you have? i assume spokes 10mm either way is not going to work but 2or3???
1-2mm but even then you can run into trouble. You can get longer nipples if too short, but if the spokes are too long you risk running out of thread before finishing tensioning.
Too short definitely better than too long for the reasons mentioned. I'd not want to be more than 2mm too short though (even 1mm too long may be too much). Best to just get the right length really.
They come in 1mm increments but most shops only sell even numbered lengths. I use the Wheelpro spoke calc, round down to the nearest whole even number and I've never had a problem.