With a secondary line, first they take the credit hrs toward your blg. If you bid say 15 days of vacation, your blg is automatically 90hrs for the month. Same applies for lesser hours, so if you have 7 days, 42hrs. As for the build, you have to put in your request depending on how many hours you have left over. I had 7 days in March, I got one 16hr trip and they bought me up the rest of the way. I asked for the trip I got, and I figured I would get more than what I did, but they bought it up. If not, you can always put it in your request that you want certain days off(my limited experience, I have gotten what I asked for).

HJB