Chad,
It has been a while since I looked at your DB and I can't find a copy. I don't have time right now to re-figure it out but here is how you calculate waste:
Divide the amount of stock used by the amount of stock you started with and subtract 1 from that value. You may have to multiply the entire value by -1 to get the sign right (I assumed that a loss was negative). If that is true, you get the same answer by subtracting the original value from 1.
So: "(stock used / original stock amount) - 1" should give you the correct answer. Or you can use "1 - (stock used / original stock amount)". I tried it out with values that I could calculate in my head and it gave the right answers. For instance, I got the total slit width (I think) equal to 7.5, got the master coil width (I think) equal to 10 and the calculation showed a 25% waste, exactly what I expected. When I increased the master coil width to 15, the waste was 50%, again, exactly what I expected. In my mind, both of those answers are correct. If you need the exact formula I used, please see my first post in this thread as my mind was freshest at that point and totally dedicated to your problem. Right now, I'm working on some other problems that have me distracted.
I guess the real answer to your question is the answer you would give to someone who asked you how you calculate waste with a manual method. The answer is about the same here.