Thank your for your detailled useful feedback. I will add some options to let users select KB or KiB and an explanation that for example when you buy a drive it says 1TB but thats not 100% of what you can use.
Also I will add another output value where these parts for filesystem overhead/reservation are taken into account.
Edit: I updated the calculator, I think for the users its the easiest and fastest to just check what size the file explorer shows and enter the amount here (instead of manually calculating the 1-4% filesystem overhead). About the stagger things, I dont know how to implement that properly to be honest.