How do I specify different variables in a model?

[excelos]

Hello!

I have a model of the progression of a work based on different stages, streams, etc which is quite complex.
You input the productivity and it outputs the end date based on that productivity.
Now I want to do the opposite, ie specify the end date and find the minimum productivity that can achieve that.

How can I do that? It's not a single function, but a cascade of functions that distribute the workload in weeks up to the end.

I assume I can write a function that will specify that the specific date, the workload must be the first zero.
But how can I approach this to give me the minimum productivity?

Thanks!