Thank you muchly PeterCanessa, I really should have been able to figure that out! Tsk! *wanders away to go find old maths books* :-) Ahh...I just tried that, but because it is a cube that is path cut (perhaps?) I found that I need to half the dimension eg: I have a 10m cube, hollowed to 95% using the 5% theory the wall should be 0.5m - but in actual fact it needs to be 0.25m! - So it becomes 2.5% of the original prim width. Thank you for making me exercise my grey matter ;-)
As a relative newcomer to building - I'm struggling to get my walls exactly the same thickness as the 'wall' thickness of the hollow cube I am using for corners...It suddenly occured to me that some mathematical genius MUST have worked out an equation for the ratio - would anyone know if there is such a thing? Or...even a list of the ratios?