Calculation when starting mid-month

Hi all

Workbook attached

I have made a workbook that my learners use to keep track of their Off-The-Job training. They are expected to complete 20% of their weekly/monthly time as OTJ Training.
The issue is, if a Learner joins me on the 20th of the month, the workbook is expecting the learner to still complete 100% of the months' training time, rather than as a % of the month that remains.
I would also like to be able to do the same for their final month - I am happy to add a cell where if "yes" is entered it is confirmed as their final month, and so calculates the OTJ Hours as a % of this final month. This will not be needed on the first month, as the first will ALWAYS be Month 1, whereas the final month could be anywhere from 12 - 24 months.

I am more than happy for the calculations to be placed in a hidden cell