Calculating hours worked less break

I work different length shift some of which are over-night.
I'm using the formula =IF(((C1-B1)*24)<0,(C1-B1)*24+24,(C1-B1)*24)=IF(((C1-B1)*24)<0,(C1-B1)*24+24,(C1-B1)*24) Where cell "B" is start time and cell "C" is finish time. This calculates hours worked whatever shift I'm on.
Is there anyway I can also deduct a hours rest break if I work more than 7 hours ?
Any help would be appreciated