To measure offset:

Measure the width of the wheel,

Using a straight edge, measure from the back of the wheel, to the flange where you would mount the wheel.

Width of wheel(mm) = W
Measurement between back of wheel and mounting flange(mm) = M
Offset (mm) = O

O=M-0.5W

Now, most fords are 108pcd a c/bore of 63.3 and a 35-38mm offset, unfortunately I forget what it should be, odviously this may alter slightly dependant upon width of wheels, to get them to fit in the arches!

Steve