"Rate" needs a "per".
"Amount" is a number of distinct units.
R = Production Rate = 12 Units / hour
H = Hours Available = 7.5 hrs
A = R * H = Production Amount = 90.00 Units
If we increase the Production RATE by 1 Unit / hour <= Notice how I did not say "one unit".
Production Amount = 7.5 hrs * 13 / hour = 97.50 -- We cannot measure marginal expenses based on the increase of 7.50 units. We need an increase of ONE Production Unit.
If you wish to define the Rate as 10,000 Units / day, that is fine, but increasing this to 10,001 per day is not helpful if you are studying production amounts on a weekly basis.
One must be consistent in the definition. Marginal Expenses are measured on the increase of One Production Unit - with reference to the accounting period that is also consistently defined.