There is a project I've always wanted to do if I only I understood programing better to figure out to proper way of doing this.

We've all seen DoF charts, I own a few myself and understand the concepts. Between formats (sizes) DoF changes accordingly. If my charts are correct it would appear the correlation is easily computable and equivalent between formats. Example, my charts say:

35mm Film Standard 1.5, 50mm lens, subject @ 20ft = DoF of 5' 11" @ f/2
4x5" format, 176mm lens (Horizontal Equiv.) @ 20ft = DoF of 5' 9.3" @ f/8+1/2

My question is, is there anything else I'm not considering? To me it appears the DoF falloff is different in larger formats, and it is this characteristic that attracts me most. Is this just me or is there an explanation for this? The charts would leave me to believe DoF should be identical but to me it appears something is different.

Any helpful feedback would be greatly appreciated. Thank you very much for your help!