Back fusing

[UKMeterman]

Go and look up the magnetic force on conductors when carrying your expected fault current

 

[westward10]

If your calculations say 35mm is adequate then that is fine, this was my point in the first place to do these calculations although your original thread stated 25mm.
Whether I would connect from the switch-fuse was a personal opinion as I realise you needed a solution to enable another supply.