My initial query was to determine a way to replicate saturation, except by eye, when one alters color space.
I normally work in ProPhoto until I save an image, in sRGB, for print or web display.
Altering the luminous RGB numbers is easy but matching saturation is more of a subjective thing and that presents the enigma.
I may not be understanding your issue since there shouldn't be a change to saturation when
converting from ProPhotoRGB to sRGB for web viewing unless you're talking about viewing the sRGB tagged image online in a browser that's not color managed.
Now, I have seen slight hue/sat shifts of certain saturated colors like intense greens, yellows, blues and light oranges converting from ProPhotoRGB to sRGB but only on my display. The only remedy was to convert to my "sRGB-ish" custom ColorMunki display profile so only I can see it.
It's impossible for me to see this on any other display used for web browsing because I don't view the web on smartphones, tablets or off the shelf non-calibrated computer systems. I only use my calibrated display.
I have viewed my images on a 98% AdobeRGB gamut Samsung AMOLED Tablet hooked to wifi at Best Buy, but the hue/sat was so off that attempting to match it was pointless. Even switching to Basic Mode that's suppose to track colors to sRGB using the Samsung's Adaptive settings was still off but not as much as Cinema and Photo Mode. The color temperature played a major role in preventing a better color match in Basic mode.