Is that the MK II Artist's Viewfinder app? I've been using it for years.There is a phone app called Viewfinder which allows you to set the film size - including 10x8 and the lens mm to give you a preview of what you will get includes a light meter as well - it takes a bit of fiddling to add your lenses and film/sensor sizes . It’s a bit tedious looking at it indoors but if you set it up and pick an outdoor subject it will give a useful indication of what you actually get without having to guess and let you target your lens GAS with less remorse ;-)