Part Qty's for Supply Allocation - It doesn't appear to make a difference on the output when I've included or excluded it.

Lets say the formula for a part is ((this_part.Width*this_part.Length)/144). Do I also need to include this_part.Qty into the formula?

No, the supply will automatically be calculated for each individual part on the order. So, if there are 13 of this_part, it will be run 13 times to check each part and then be added together.