Formula to calculate the FTE Month on Month

Hello Team,

Please help me to find the formula to calculate the number of days between dates.

I have Project start date and Project End Date against individual names. Each person's start date and end date differs. i am trying to find how many days he is assigned to the project month on month. If he is completely assigned for a month which is 30 days, then its 1 FTE, if its 15 days then 0.5 FTE.

So if the person is assigned from jan 1st 2019 to Feb 15th 2020, the result would be 1 for all month from Jan 2019 to jan 2020 and it would be 0.5 for Feb 2020.

So based on the start date and end date i am trying to find a formula that will calculate the number of days the individual is assigned against each month. Attached is a sample data and result is updated manually. Please help.