Why per-hour beats per-part
Two items with the same profit are not equally good if one prints in forty minutes and the other in nine hours. The slow one occupies the machine that could have made a dozen of the fast ones, and that is invisible until profit is divided by hours.
This is also how you compare a cheap trinket with an expensive commission without arguing about taste.
What quietly eats the margin
Time you do not bill: preparing the plate, removing supports, sanding, packing. It is the most common reason a workshop stays busy and ends the year with nothing. Put your hourly rate in above and watch the cost move.