problem calculating a cell based on the specific content of another cell

I am designing an order form and need to calculate the quanity of an item to order based on min OH as well as current ON

Cell A = qty min on hand (this is a sum of a range of cells)
Cell B = current on hand quanity (this is a sum of a range of cells)
Cell C = order quanity


So lets say I want to maintain a min 5 units so 5 is in A
I add that I have 2 units currently so 2 is in B
Cell C adds A - B and populates 3 in C as the order qty

This works great except that when the form has not been used col B is showing 0 as the range of cells it is adding is blank. As a result the max of 5 is being ordered in cell C instead of zero as it should be.

What I need is the above equation works except when b is blank in which case C will equal O

I can email a copy if someone can help me out but can't understand what I am saying here

mike