Negative Resource availabilty

lanemark

We are starting to use the dispatch board to allocate resources (service engineers) to service orders. We currently have a resource capacity set for each resource at 8 hours per day, but on a lot of occasions we need to allocate more than 8 hours, but this sends their availabilty to negative.
Is this the normal thing? should we increase the availabilty on that particular day to match the requirement, or leave it negative, but this may affect some reporting later.
Is there a way to prevent a resource being allocated if there is not enough availabilty, or at least a warning.

Alan