offset help

right this is my maths.

(rim width/half) to find the wheels centre. then plus or minus the offset to find out where the rear face of the wheel would sit. by that centre line

so.

(rim width/half)+?

then measure the resulting number from the hub face out to the arch and see where it lies..... is that correct?

ie a 245 rim with an offset of -33 would sit 89.9 mm away from the hub face.

:detective:

i think im right its just i cant spell very well and the maths isnt exactly top notch neither.