You can't. The way this wiki treats durability is energy-based, and you can't get work without distance.
The proposed solution of using potential energy above is wrong, and ignores some basic logic.
In real life, the ability not to be crushed by something is related to
compressive strength.
On the wiki, we just lump this into your "Lifting Strength". For example: if someone has Class M lifting strength, then we just assume they can't be crushed by Class K telekinesis.
To put it simply: your lifting strength already accounts for your ability to resist being crushed, but doesn't help your energy-based durability.
If the boulder was dropped onto them, then we could calculate it. However if it is placed gently on them, and their only feat is sustaining its weight, then you can't get a durability value, only a lifting strength value.