As noted already:
1. Measure you own ERD - typically referencing the screwdriver flat of a standard profile 12mm brass nipple. Rim rods work well for this using the referenced nipple...
2. Round down the drive side result after calculation.
3. Keep the non-drive result as-is OR go up at most .5mm or down .5mm - whichever most easily gets you a whole number.
Getting the spoke length you really want is easier if you have access to a spoke machine - but only a few folks have that option.
Be careful with factory boxed spokes - sometime they'll be .5mm shorter than what the label on the box indicates.
=8-)
__________________
5000+ wheels built since 1984...
Disclaimer:
1. I do not claim to be an expert in bicycle mechanics despite my experience.
2. I like anyone will comment in other areas.
3. I do not own the preexisting concepts of DISH and ERD.
4. I will provide information as I always have to others that I believe will help them protect themselves from unscrupulous mechanics.
5. My all time favorite book is:
Kahane, Howard. Logic and Contemporary Rhetoric: The Use of Reason in Everyday Life