Take measurements and do the maths. Measure the overall width (from insides of the lips), and measure from the inboard lip to inside of hub surface. subtract half the overall width from inside lip to inside hub. if you get zero that means the hub surface is exactly in the middle. Do it in millimetres. negative value means its a wide offset, positive values are a narrower offset. Eg; if the answer is 20mm its a +20, ect. if that doesn't help just google it and get a diagram.