John, I know this is not a great deal of help, but maybe insightful. The three sets of 27" rims I've had laced to high flange hubs (Campagnolo Record, '62 Normandy, and mid '70s Schwinn Approved), my LBS had to use custom cut and threaded straight gauge spokes for both a 3X and a 4X pattern. Fortunately for me, they did not charge extra to cut and thread the spokes.

If memory serves me correctly, I was also in the 300-310mm range when I did my best to determine the spoke length (something I'm not that good at). I couldn't find these lengths as well. Maybe this is why the LBS had to cut mine.

Best of luck.