Analytical Geometry for Unity

I have a project for creating greeting cards. X and Y positions in pixels are unavailable. Width and Height in pixels are known X and Y are in points. So i do Analytical Geometry transformation to know the X and Y in pixels. But it not precise because X for thin image is less then X for big picture also for Y. I need some one who knows the math to fix that the pictures will be incorrect position. I use Unity 2017.1.0.f3. Another versions will not accepted!