Need help with measurement to rebarrel my wheels...

I'm currently running Venti VP 106 wheels 20x8.5 at +38mm, I found a guy selling 2 barrels off Venti VP 101 wheels that were 20x9.5 unknown offsets. However we do know that my wheels have 2 inch lips from the face mounting point and his have 3 inch lips... is this enough information to calculate a full inch of negative offset (I'd be sitting at +13) or am I way off? Any help would be appreciated & thanks ahead of time.

20x9.5 +25 

Just enlarging the outer lip 1" will only change the offset by 1/2 an inch which is around 12-13mm.    Therefore 38 - 13 = 25   

That is what I came up with using what I know about rims.   Double check with others and then confirm.

^Thanks much Blitz... I got more info as well, found out the offset of the barrels was +35 before he took the faces up and they had a 2.75" lip...

the original specs of the donor wheels aren't important. if the new lips are 1" bigger than the originals than blitz's calculation is correct.

I really appreciate the help, good looking out...