How frustrating, I think when I was 11 years old I could have done this in approximately 30 seconds, now more than a quarter of a century later my brute force methods lead me to a horrific looking equation that can't be optimal!. Please check attached image, this is the projection of an image at one distance (100cm) viewed at another distance (160cm), I need to find out the "measured" size either side of the centre of the projection based on a presumed "tilt" at the second measuring surface. I am sure there is some kind of triangle based wizardry that makes this a simple matter to express the (unequal) two distances as a ratio to their (equal) values in an untilted world. Thanks for any help