By diffraction, I assume you are talking about sharpness of the image. DoF is another aspect of sharpness.
First, photography as we think of it as a creative pursuit is about creating pleasing images. Pleasing is a technical term and basically states that the output is subjective--it is the observer's perception and reaction to an image. Another important point which is often lost is the photography is not comparative--the image is judged on its own merit. Comparisons are done to show changes, but images are presented individually. So when we look at an image, we want to perceive it as sharp. It does not matter if another image or set of conditions is sharper.
Sharpness is a subjective quality as it lies in the observation of the image. It is not an absolute quality in the object. A meter is an object quality--it is either 100cm or it is not. The observation and outside conditions do not change that. Sharpness is a perceived quality based on many outside conditions like viewing conditions and the eyesight and taste of the observer. So the concept of viewing conditions arise when talking about sharpness. The standard viewing condition in the simplest form is to view an image from a distance equal to the diagonal of the area. This is a nice definition as the actual size of the image is irrelevant--the larger the image, the greater the viewing distance and so the perception of the image is always the same--a 300dpi 8x10 viewed at 12" looks the same as a 150dpi 16x20 viewed at 24" (you notice the number of pixels do not change).
A simple definition of sharpness and DoF is to do with the smallest permissible circle of confusion that is perceived as sharp. This value is also subjective and does vary both with camera/lens manufacturers and photographers--if you don't like one set of numbers, you can make your own. Zeiss defines the permissible circle of confusion as 1/1500th of the format diagonal. DoF is related to format, not pixel pitch, as it is in relation to viewing conditions that assume a particular distance an image is viewed and in relation to the human visual system.
Now folks will say they don't always look at an image at standard viewing distance. But here is the neat thing, the illusion of sharpness is robust and has a built-in error. The definition of a photo-quality print is 300dpi, assuming an 8x10 print viewed at about 12". To keep the math simple, than means the diagonal of the print is divided into pixels that are about 1/3000th of the image diagonal. The definition Zeiss gives for the permissible circle of confusion is 1/1500th of the diagonal. This means if you view an image at half viewing distance, the plane of focus should be acceptably sharp--a 16x20 viewed at 12" is still sharp even at 150dpi.
MFD easily exceeds these numbers.
So, the CoC depends on the format diagonal. Sharpness is a function of the format size and viewing condition.
I believe you are wanting to know how far you can stop down to maximize DoF without diffraction becoming obvious. With a 6x4.5 format, f/22 or f/32 should be the diffraction limit. Your personal diffraction limit may be different. I routinely stop my 645D and P25+ down to f/16. I sometimes use f/22, but that is getting o my threshold. But it is not a simple thing, high-contrast scenes work better and unsharp masking makes a huge difference. When I got my camera I did some tests. I shot some usual condition all the way down to f/22 and made 36" prints form them. The benefit of the added DoF certainly outweighed the impact of diffraction--sharpness is not what you see at 100% when comparing it to other conditions. I would urge you to test yourself to see the results, and by results, I mean going to a print.
Now, as much as I respect my fellow photographers, we all suffer from too much experience. We can see the defects in an Ansel Adams print from 200m. We have come to learn to recognize very small changes in the image. We obsess over individual pixels. But the audience does not see nor care about the minutia. What they see is a great, sharp image. I have never had someone walk up to me and ask about the diffraction in my print.
I hope that answers your question.