I made and sold wire wrap jewelry, chaine maille jewelry, and things like hand made stained glass kaleidoscopes for a lot of years. A large number of things, especially kaleidoscopes, were 'bread and butter' or I made a lot to fill the display. Sometimes it might take a year or so to get to the stuff from a large base run so. I would take a recipe card and jot down start and stop times for all the time I put in on that batch through the base run (say 100 at a time, for kaleidoscopes, about 85% of the work was the same so I would put in the time to run 100 at once. This saved on setup of equipment and jigs time.) Then as I finished each one I could note the time for just that part and take the previous lot information (sorted to how much time and how much the supplies were, hard and consumables, and add the overhead for the entire batch, then break it down to how much 'per unit') and work the final net cost.
Even if you do a process and it's a year later before the next bit is done, you can still use the card method. If 100 pounds of X becomes 65 pounds of Y, you still have the breakdown of what all the materials X cost, and the consumables used to process X, plus your time, plus your overhead, and reduced to a per pound cost for Y. Then you use Y to make 8000 yards of Z, you can document the costs for Y to become Z, and figure a new net. Then figure your selling price and what
profit there will be.
The recipe card method has been working for me for about 30 years. I do take pictures of the
cards and turn them into digital storage that way, but.
Also try to utilize 'zero time'. this is time you are doing something else. Like fairly engaged in TV or computer, or sitting in a doctor's office waiting for an appointment or for someone to finish an appointment, etc. Really tedious things like foiling small stained glass pieces or chaining beads on wire, I did a lot of that while reclaiming zero time. It's surprising how that can add up.
One other trap is 'replacement cost'. I did a lot of work when silver was $5-7 an ounce. It's not that now. I had to reprice to replacement cost. This meant looking up current prices and refiguring with the help of those
cards, what went into a piece. Then retagging. Good luck on your math.