Inventory; how to reset input variable without changing the output cell's value

I'm working with excel 2008 on Mac OSx... I have a working formula set up where I can input the number/type of circuit boards I've made, and it will automatically deduct the components used from my "component inventory".

So far the formula works great; now how do I make the changes permanent, even after I reset the input variable (number/type of circuit boards made)? As it is now I have to keep a running tally of how many circuit boards I've made, which I imagine will get confusing over the years.

Also, to keep the running tally, I have to add how many units of that component I've recently bought to the original stocked amount used when I first made the system.
I.E.- I start with 500 capacitors. Each GPS circuit uses 4 capacitors. If I've made 25 GPS boards, the equation looks something like this: =500-sum(25*4), leaving the cell to display: 400 capacitors. But the equation still has that fixed '500' number, and if I eventually buy 200 more capacitors, I have to change that 500 to a 700. Now if I want to reset my input variable (the 25 in my example equation)... my entire inventory is off. Will I forever be stuck adding on to the fixed and the variable numbers?

Attached is my excel file... the only cell acting as a formula is 'B4'.... with it's inputs being cell C52-C54, each of which use a different amount of the component represented by B4 (220ohm 1/2w resistors). The formula is as follows: =500-SUM(C53*3)-(C52*7)-(C54*1). I need to delete the information in C52-C54 yet keep the value of B4 the same. I also need to be able to add/subtract from the component inventory without ruining my equation.