In direct answer to your question the most universally accepted standard is 2 times the pixel pitch (IQ180 pixel pitch is 5.2 microns). When juggling zeros converting from microns to mm to cm (depending on what your calculator asks for) I suggest a simple
google search. It's not the most powerful unit convertor out there by any means, but it's fast and easy to use.
I find it more useful to actually start with the end product (once) and reverse engineer your own implied CoC (to use henceforth).
Start at infinity and take several images backing away from infinity, make a composite file with a small part of each file (as it would look printed at your normal print size), make an actual print, and judge the image which is the last to still show acceptably sharp detail at infinity and then use trial and error on your app to determine the implied CoC that agrees with that result.
The reason I suggest this is that "acceptably sharp" is the key to the CoC. This will vary based on the type of paper you use, your preference for print sharpening, viewing distance, lens sharpness, diffraction limitations, level of scrutiny, and may be influenced by more practical matters such as compromising very small amounts of infinity sharpness for a more flexible/practical foreground DOF.
Of course this requires some careful notes (I suggest just taking an iPhone picture of the location of the focus barrel - one for each raw file you capture, it's easier and inherently more useful than trying to record a specific distance) and a solid hour or two of work, but you'd only have to do it once. The resulting implied CoC should apply to all lenses at that f-stop and print size and level of scrutiny.