How to calculate the thickness from an inset to a certain number?

[anon32682146]

Once again I turn to this amazing Blender community.

I have a little problem with calculating the caliber of a certain tank gun. the outer diameter of the gun is 138.741 mm, and I want the caliber to be 75 mm.

I have a flat face at the gun’s front, and so far I’ve tried to inset the face and get it to 75 mm in diameter, but I always end up with a different size.

I’ve basically with my low-end math done 138.741 / 75 = 1.84988, but that gives a diameter of approximately 89 mm…

Any help is greatly appreciated!

Thanks in advance

[Okidoki]

The use if the inset “thickness” is always… weird… better do this:

Having a disc at the end of the cannon; extrude in place and then scale to 75 / 138.741; then extrude this disc inwards…

[anon32682146]

Worked a treat, Okidoki! Cheers!