Depending which way you are going. Assuming +15 and +10.
+10= 0.39" +15= 0.59"
Now, if you are dealing with inches, you are probably dealing with an American wheel manufacturer, which doesn't measure using offsets. They usually deal with backspacing(from face of the wheel closest to the chassis to the bolt surface). So ultimately you need to know how wide the wheel is to figure out the backspacing in inches. And since I'm at work, here ya go:
what i am trying to do is have a big lip but not to much poke. my plan is 15x8 diamonds w/ a 5'' back space (et 20?) in the fr. and a 4'' backspace (et 10?) in the back does that sound right thanx joel
With that setup, it'd be like a +25ish in the front and a +0 in the rear. One inch equates to about 25mm. So we are looking at a 8" wide wheel, which means in the front, with a 5" backspace, you are going to move the mounting surface one inch past the centerline of the wheel, being +25. In the rear, with the same width, moving halfway through the wheel(4") makes it a +0. I would think if you are going to run a zero offset in the back, you would want a little less offset in the front, maybe make it like a 4.5" backspace. It depends how much you can fit. That's really the big question. There are offset calculators online, so you can put in your current specs and then what you want to do, and it will figure out how much more/less clearance you will have on both sides. In fact...