Hey Marty - heres an example of the rulers ( pink arrows pointing at them ) and the guides you can use to square off/ measure a certain area. You go to View and tick Rulers and they appear - then you put your layer tool ( top right tool in the toolbox ) on the ruler , click and drag out the blue guides.Thats how I do things and I am happy to explain stuff to you by email if it helps you at all.
Now - this aint the perfect picture to work from as it is on an angle - this is just giving you an idea of one way to use P'shop. What I do is physically measure that square out on the PF ( around the 2 triangel ) and then make my Photoshop image at those sizes - This is not a perfect example as the image was taken on an angle - just hopefully gives you an idea. I use it and it works OK. There may be better P'shop advice out there somewhere
