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Might be wrong, but I believe length of thread in nipples of varying sizes does not change
Some do, some don't. The internet suggests that DT use about half the extra length for extra thread. The trouble with using long nipples to compensate for short spokes is that you run into the issue of not engaging the threads in the nipple head, which some people suggest leads to the head breaking off at the join with the shank
Use dt swiss 16mm nipples which effectively will add 2mm to your spoke lenght so you are good to go