Originally Posted by Bill Kapaun

I used the numbers you provided with an ERD of 578.
Spocalc came up with 269.7.

Personal experience using the Sun CR-18's (and Sun CRT 16 II's & M13 II's) would cause me to round up the 1.3mm
I would much rather insure the spoke end reached the bottom of the screwdriver slot of the nipple than be short at all. (short results in a weaker joint)
You still have 2+ mm of thread left before you are flush with the end of the nipple.
Even if it protrudes a bit, it's a non issue with a double wall rim, as long as you don't run out of threads.

I've used Suns published ERD's, so maybe that's where my extra mm comes in?
IF you've carefully measured, I can't argue with your results.
You did measure to the bottom of the slot????

BTW, since this is a drum brake wheel (I'm assuming powerful braking) I'd go to a 14/15 double butted spoke to help absorb the stresses. The thinner middle section "stretches" a bit, resulting in less flex in the bend. Less fatigue over the life of the wheel.