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.