off-set wheels on forester?

anybody know the off-set for my stock 15" wheels on a 98 Forester L. on the rim it reads T 15x6j DOT 7 97 J6ER4 and read ER near the lugs. There is also no mention of Off-set in my Owners manual either. I use this vehicle off-road an have a 2" suspension lift. the lift allows and extra 1.5" of tire hieght clearence but still only have about 1/2" width clearence between the sidwall and the strut. I would like to put on custom wheels that are off-set -10mm from my current wheels to allow for a 20 mm wider tire. I'm running 205/75r15 now and would like to put on 225/75r15's.

What you could do is use the same offset and get a spacer plate to put between the hub and the rim.
We used to do that in SA when we were going on trails with a lot of sand and wanted a wider track.

I do believe they are a +48mm.