Hi. I am printing some exhibition pictures on A3+ paper. The images are all landscape, but not necessarily exactly the same size. In order to make them look good as a group I have decided to center them on the paper and make the gap above the image and below the image 40mm. With me so far? OK. I have saved one of these images after printing, and can use it for starters when doing my next print, BUT because the next print might be slightly wider or thinner, when dropping the image onto my saved template it does not necessarily measure 40mm above and below, it could be 42.3 say or 39.7 for instance. Now here is my question. How can I easily adjust the size to my preferred 40mm above and below letting the side spaces adjust themselves? If I try putting 40mm in the top space box, the image just moves up or down and the bottom space is left at not 40mm. I am not allowed to enter 40mm in both boxes as the software prohibits this. SO... my only solution is to drag a corner to make the image larger or smaller depending on which will get the same to the right height and then hit 'Center' and see if it ended up at 40mm top and bottom. But this is really hit and miss although sort of achievable in the end after a few goes. Is there an easier solution that I am missing here? 

Brilliant Terry. The second part of your reply does exactly what I wanted. I would never have worked that out on my own!