You're going to have to give more information than that. What arc are you setting? What are you trying to do with it? If you make ANY changes to the DefaultCore.txt, only people with the same file will be able to play on a D3 server that uses the modified specs. Also, you can only use certain arcs, anyway (i.e., only 30-, 60-, and 360-degree arcs, and only in certain directions).
If you've already seen my spec mod tutorial (I'll bump the thread), then perhaps you can describe what you want to do with this so I (or someone else) can offer suggestions.