Probably by testing. Nobody usually wants to flow gas inefficiently through a CMP, so not any method, or even data, out there that I know of. Maybe they do that in mining work.. maybe.
From "BigInch's Extremely simple theory of everything."
It's old, like me, but "This bibliography is the result of an extensive literature survey in an attempt to obtain detailed information, both empirical and theoretical, on the pressure drop occurring in corrugated hoses, bellows, and gimbal joints":
I did better than I expected. The old document used the Fanning friction factor. The one at the url below has been converted to the Darcy-Weisbach friction factor. My company and I have used this correlation for years and years.