I'm huge into measurement, and quantifying this has stumped me. It's one of the few areas I'm willing to surrender and say "Let's just pick a % of time to put on it."

It's bad to say "Let's give it to folks who are underutilized or have capacity" because those are rarely the people who can do it well.

All I can come up with is the hardening % should be in proportion to how catastrophic a failure is, while keeping some faith that well done hardening ultimately pays for itself.

Philip Crosby wrote about this in manufacturing as "Quality is Free" https://archive.org/details/qualityisfree00cros