Calculate Between Two Times With Additional If Calculation

Hello folks,
Glad i am here and hope you are all keeping well?
This is of course my first post and as you have probably guessed, i am looking for a little advice if possible please.

I am trying to create a work shift pattern from Sunday to Saturday, Weeks 1 -100, and each cell there will be text added such as "A,M,L,N,R" etc. Each letter represents a specific shift.
I have created a table for the shift pattern and another table for start and finish and total shift length times.
I am trying to create a formula where i can work out the time between the end of one shift and the start of the next day shift. ( this i can do ) but the difficulty i have is when i hit a rest day "R"
Is there a way to create a formula that will calculate the time difference between the end of the first shift and if its a "R" shift adding 24 hours to the total or even 48 if there are more than 1 rest days?
I am not sure if its even possible, however im sure someone will have a workaround
example below Sunday the shift finishes at 17:30 so would need a formula to calculate from 17:30 until Wednesday 07:15 so i can display the amount of time off between shifts

Sunday Monday Tuesday Wednesday
M R/D R/D M


Looking forward to any responses
Thanks StokieWorm