Image offset is expressed as a percentage. It can be 100% and more, namely when the center axis is outside the image area. If the projector is mounted on a table, the image offset refers to the lower image boundary; if the projector is mounted on a ceiling, it refers to the upper image boundary.
If the projection lens is at the height of the lower image border for table projections, then the image offset corresponds to the entire projected image. It is therefore 100 %. If the projection lens is below the lower image boundary - assumed 10 % below the lower image edge - then the image offset is 10 % plus 100 % of the total image height.