Is MTTD a useful metric? How do you calculate it?

Context: SOC analyst. We are being asked by the new CISO to calculate Mean time to detect.

This seems extremely hairy to calculate for all of the incidents we get. My only idea is to have a field where analysts have to manually enter a timestamp when the activity related to the alert started, so that we can subtract that from the time the alert fired off to calculate the time to detect.

We have a SOAR platform, but not a SIEM so if we wanted to automate it it would be a mess trying to pull from all the log sources.

My question is are: is this metric worth calculating (maybe just for true positives)? Do you calculate it and if so how?