The spoke threads bottom out on the nipples and the wheel won't tension. The spokes are too long, right?

I used the spocalc spreadsheet and two online spoke length calculators - same length calculated by them all using published data for the rim and hub in a cross 3 design. (Salsa Semi 29r Disc and Surly New Disc front). Rounded up to the nearest mm (so says Sheldon) and ordered them.

Built the wheel and attempted to tension it. After looking at a floppy wheel for a while I measured the six spares I bought - all were 2 mm too long. I measured these from the center of the head of the spoke to the end of the threads. Then I measured the uninstalled set I bought for the rear - every spoke is 2 mm too long.

The cumulative error is 4 mm across the wheel diameter. Is the length that critical? Perhaps I should have rounded down rather than up (what the DT Swiss calculator does).

My spokes were cut to order using a DT Swiss spoke machine. How closely does the length of the DT Swiss boxed spokes compare to the indicated length?

-- Bradley