X-Com Files actually makes Stamina recovery dependant on Health, not TUs (following Piratez as an example). Which personally I consider a more logical approach. Yes, if you get high TUs and low Health, the soldier will tire fast, but that only means you need to send them to a gym or something. Additionally, it actually depends on current Health, not maximum Health, which means wounded characters get winded much faster.
Here's the code for it (in armours), works only for OXCE and OXCE+:
recovery:
energy:
flatHundred: 0.1
healthCurrent: 0.25
I'm fairly sure you could do something with TUs instead of Health, if you'd like.