I think you are mixing up two different specs.
OFFSET is the distance from the inside wheel mounting flange to the geometric centre of the wheel and can be
POSITIVE or
NEGATIVE in value.
As a result of the above there is also
BACKSPACING, the distance from the wheel mounting flange to the inside tyre mounting bead.
So, the backspacing can vary with a constant offset because of various rim widths
Anyhoo, a 6" (152mm) wide rim, with a 30mm positive offset, will have a backspacing of 106mm [(152/2)+30],
JUST enough to allow a 205/50 x 15" tyre to clear a 200B strut in a 1600