Yes I thought you might have 16" wheels on your truck. However I would guess you have a taller sidewall than a passenger tire 225-60-16....also consider my toe spec is 0.10 deg. (read one degree). I do not know the spec on your truck. If 0.10 deg. you are probably right on @ 1/4" depending on where you are measuring. The mfg. spec is for the center of tire at tread diameter. A taller tire will account for the difference.
Now if you lay it out on paper you start by laying a line the length of the radius from which you are going to measure on the vehicle i.e. (center of wheel to front lip edge) Mine measures 8 5/8" or 219mm. Then using a protractor centered at the start of the line draw your deg. line out then measure the distance between lines 1/8th inch = 3.175mm.
I don't think it a good idea to use tread as a measure point due to possible run out in tread design tire to tire. This is merely an opinion on my part