I’m not sure how the damage it takes is calculated. But one thing I do know is that the damage enemies do to themselves from Reflect Shell is ~6.66 times the damage they actually do to you (to make it easier, just divide it by 0.15.)
Speaking of that, I found out my estimate of doing 0.15 the damage of the reflect shell was spot on correct since I found patch notes that show the Elite Drone Claw was nerfed down to 35 damage per bullet (from 40 and this is Beginner) and since the damage they do on Master with normal modifiers is 6.75x the amount of their Beginner damage, I found out that the 1575 damage they do to themselves is equivalent to 236.25 damage per bullet (so a 5 shot down) and when converted to Beginner damage, is exactly 35.
Finding that out was amazing since it showed that they used that exact way to calculate Reflect Shell damage for Gunner.
Might have gotten carried away there but that information can be used to possibly tell how much durability the Stick loses after taking some hits (Hunter Markzas would be good for this.)