Absolute strength testing and tendon injury rehab
Measuring the injured part to the uninjured part is not adequate for determining return to sports status. In runners for instance, patients with achillies tendinopathy are weaker in absolute terms relative to their peers for a given task like heel raises.
If you can barely do a single leg-heel raise on either leg, cutting your running volume way back to where your symptoms become manageably stable is a sensible first step, and then proceed to load the muscle to achieve strength optimal for the demands before stepping it back up again.
This is one of a few reasons why tendon complaints remain chronic. Measuring the difference between your left and right sides is feeding false information. You may need to understand just how high the bar needs to be set before it can get better, and have a reliable and meaningful test that compares you to your non-injured peers.