Call center employees: need advice regarding work situation?

I have been working a new call center job for about 2 months now and I've been volunteering time off the past 3 days due to it being really slow. We have what is known as unavailable time (or aux 9 time) where you are able to use 55 seconds per each hour you work for bathroom break/ breather between calls. I've been using the full 8 minutes of this time within my first 4 hours of working each day and then getting sent home early about halfway thru my shift so I end up using too much for the day but I didn't know how this would affect my occupancy or even if it would. how should I explain my lack of thought on this to my supervisor when he talks to me about it next week? I have one more day to average it out.