DT has a spoke calculator on its website which will show you the calculated length and the DT recommended length. It looks as if the DT recommended length is always to round down to the next mm. The calculated length is the same as the caculated length from the widely recommended spoke calc spreadsheet and the same as manual calculations using the formulas in the Brandt book. No surprise there. In contrast to the DT approach, the Sheldon Brown website says that his preference was to go to the next available size greater in length. Tolerance is nice to have, but not all that important. Round up, round down, you'll be fine.