I will try and snap some photos of my wheels/fenders. I run a 20" with a 275 in the back.
Measuring offset is not super easy, buy my guess is the wheels will be stamped with it somewhere on the back, either on the spokes or on the hub. look for the width stamped on there and the offset will either read "+20 or it might say et20". I'm not sure if thats the number, just for example.
Then once you have offset and width you can calculate your new offset by adding the spacer
And they aren't 11" wide, my guess is they aren't 10.5". Tough to say, but based on how they look my guess would be a 10" wheel.