Most likely you measured the string wrong. You missed the measurement by 0.7 of an inch which is about 17 mm. Not a big distance, I think maybe the focal plane of your camera back is not marked or you measured to the back of the shutter as opposed to the center or middle of it. I would say dont worry and go with the 150 mm string marked in half stops.
Theoretically you should measure from the focal plane to the nodal point, but heck if you are going to get that specific it defeats the purpose of what you are trying to do which is to simplify things.