y=axb

You can use the learning curve rules to calculate the total time for any number of batches starting from zero. So you can calculate the total time for 600 units (12 batches of 50). If they then make another 800 units, then they will have made 1,400 units (28 batches of 50). So we can calculate the total time for 1,400 units. To get the time for the 800 extra units, we can't calculate it directly. It is the total time for 1400 less the total time for the first 600. All of this is explained in details, with examples, in my free lectures on learning curves. The lectures are a complete free course for Paper F5 and cover everything needed to be able to pass the exam well.