Excel 2007 : Formula help... Time calculation

I'm trying to create an excel file that will calculate my time. I have most of it figured out already, but I need a little help on the last part. Working full-time at my company, I have to be clocked in for at least 40 hours a week. My boss doesn't want us to work over time, so sometimes I have to leave early on Fridays to prevent going over. The spreadsheet I've created adds the sum of hours worked per day, deducts it from 40, and shows me how many hours I have left to work for that week. What I'm trying to get my spreadsheet to do now, is to tell me exactly what time I will need to clock out based on the number of hours I need to work on a particular day. For example, if I enter that I clocked in at 8:15 AM, and that I need to work 6 hours, what formula will I use in order to see that I need to clock out at 2:15 PM? What about if I wanted to include a lunch in that?

I've attached what I have so far. The part in the yellow is what I'm needing help with. My company rounds time to the quarter-hour, that's why there are two "Time Calculators"... One is the actual time, the other is the rounded time.

If you need anything explained, just let me know...