I'm new to the forum and looking to build 3 piece wheels from 2 piece BBS RS740 and would like some advice.

I have some BBS RS740 centers that I want to convert from 17 in to 18 in 3 piece. I can't figure out what lip/barrel combination would give me the right offset. I have a 2008 BMW 135i which needs a high offset, the desired specs are below. I'm not exactly sure how to account for the backpad when calculating what lips and barrels to order. From my rough estimate, it looks like the backpad will add about 1 in / 25 mm (see pic).

18x8.5 +45
18x9.5 +45 (or higher)

1 in lip and 7.5 in barrel in the front giving me ~6.5 in backspace (+44 offset)
1.5 in lip and 8 in barrel in the rear giving me ~ 7 in backspace (+44 offset)
Does this sound right?

I could also go down to an 8 & 9 in wide wheels if needed.