This will let you calculate the total time for the first 100 units.
The remaining units all take the same time as the 100th unit, and to calculate the time for the 100th units you take the total time to make 100 units and subtract the total time to make 99 units.
Have you watched my free lectures on learning curves? I explain and show how to use the formula, with examples, in the lectures.
The lectures are a complete free course for Paper F5 and cover everything needed to be able to pass the exam well.