Swapping Wheel Faces, calculate new Offset

[rollininstyle2004]

So I have been looking at a set of wheels, but the rears are too aggressive, and the fronts not aggressive enough, so I am thinking I can swap the faces.

I did some quick and dirty math, but would love for someone to validate that I didn't mess anything up:

Front:
Width: 9.5''
Barrel: 6.5''
Lip: 3''
Offset 30

Rear:
Width: 11''
Barrel: 7''
Lip: 4''
Offset: 12

So I took the midpoint of the wheel, subtracted the lip size, then subtracted the offset to calculate the face thickness, then swapped that size and worked the calculations backwards. This lead me to the following new offsets:

Front: et 18.35
Rear: et 23.65

Can anyone with some 3 piece wheel building experience let me know if my math is correct?

Thanks!

[zoo]

You need the measured overall width to get the exact et numbers.

[KyleAnderson]

I get the same numbers for the new offsets.

[Ollie]

Your math is correct