Looking for some info on how United calculates partial refunds.

One way ticket cost $338.25, booked JAC-DEN-PHL. JAC-DEN diverted to COS then eventually made it to Denver but I misconnected. United wanted to send me through Chicago with a 7 hour overnight layover. I canceled and bought a nonstop on Southwest.

Requested a partial refund and United only gave me back $216.96. I believe refunds are based on distance but I am not sure how they are calculated. I only flew 406 miles out of the 1,964 miles on the original routing. I feel I should get roughly 80% of my money back, not the 64% that United calculated. Are they factoring in the diversion somehow? I understand how a scenic tour of Colorado added to their costs but not sure I should pay for that.