Well, I don't have the roadster at hand to measure the geometry, and mine is buggered now anyway, but I have 2 spare brake drums. Their thickness from the backing plate flage to the wheel seat is 65-70 mm. So, if we combine your measurements and mine, ~1400 + 2 * 68 = ~1536.
And you're mistaken about the wheel offset. Sierra's offset is positive and positive means to the outside.
