I have a project I am working on for a customer. She wants to use a series of three of my pictures. One 16x24 and two 8x10's. I am currently working on the composition of the 16x24. When I crop and resize in photoshop, it makes the ppi 87. That doesn't seem very well. What should it be for a 16x24? Is there a formula or do you just pick 300 or 400?