How to calculate forwarded Week of Stock w/ MD in consideration

I already have a formula to calculate the Forwarded WOS but I have 2 problems:
- Exercise 1: at the end, when I have too much inventory and my line for Sale stop at*row 15, I don't want to have an NA as a result, I would like the formula to go back to*the beginning row C6 and on to calculate the forwarded Week of Stock
- Exercise 2: Same logic, but taking the MD in consideration.
Thank you

See attached