Calculate % Change Across 4 Columns when only some columns are populated

This is my first post so hope someone is helpful out there.

This is a piece of a spreadsheet meant to track patient Range of Motion performance over a max of 4 weeks.
How do I calculate % Improvement [write the formula for column J] to account for the fact that sometimes a patient is discharged after being seen 2 weeks OR 3 weeks or 4 weeks. I don't want to change the formula every time. 

The SUMMARY % Improvement in J7 - will be changed to the average of each individual patient % Improvement from column J.

Looks like I need a dynamic range from what I can see from Googling. Do I need to use Offset ?
Appreciate the help in advance.

Thanks for the quick response. I popped the formula into J11 but received a formula error.
Not sure why?

Thanks for the quick response. I popped the formula into J11 but received a formula error.
Not sure why?

Thanks I see you are finding the largest value from the last column with data. thanks but I still need to calculate % improvement from the Week1 column. I tried a couple of things but it didn't work.

1000 pardons -- I see you solved the equation in column J.
Many thanks for the assistance