I am working on a formula and am stuck. I have a goal date that I want to
have a certain percentage of customers migrated (90%).

There are many weeks leading up to the goal date. I have the following
criteria setup:

Prior to Week-4 2% of base migrate
Week-4 20% migrate
Week-3 20% migrate
Week-2 20% migrate
Week-1 20% migrate
Week-0 balance migrate

How can I set up a formula on a declining base to reach my goal of 90%?