300mm is very long for a small flange/tubular rim combination, so I suspect that you might miscalculated someplace. Since computers don't make mistakes, it was likely some data entry error.
First of all, where did you get the 615 ERD value? This seems very large for a tubular tire rim, where I'd expect something closer to 607 or so.
Otherwise the build looks OK, so I suspect it's simply that the spokes are too long.
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Last edited by FBinNY; 09-07-14 at 12:52 PM.