When to add another driver?

[volvopm]

We are a two manufacturer dealership. Right now our delivery area span about 70 miles. Right now we are running two drivers. One has everything north, and the other has everything south. We also have a warehouse/backup driver. Yesterday we had almost 41 stops through the day. Is there any key factors to look at that will help me say " i need to loose the extra warehouse guy, to full time delivery". Anyones input is appreciated, JP