The DT Swiss spoke length calculator has an option for nipple length. What’s that about?
DT Swiss deals with the issue in a complicated way, which is the reason their calculator asks for nipple length. Standard DT nipples are 12mm long. When you use longer DT nipples, such as 14mm or 16mm, you need to shorten your spokes. This is because DT Swiss adds internal threads as nipples grow — so you need a shorter spoke to avoid bottoming out. As a result it’s best to pretend DT nipples longer than 12mm don’t exist, particularly in the case of aluminum nipples.