Excel Time Calculation - Please Help!!

Hi There,

I am trying to calculate user avaliabilty for telephone stats..

See attached for data.

Firstly I am trying to work out the time difference. In E2 i have a formula, just simply D2-C2. If the result is positive it gives a result, but if negative just puts hundreds of #?

I need to get to the % avaliabilty. This can be over 100%, which would be the case for the second row of data where the user went over thier avaliabilty.

Any help would be fantastic!! Thanks