In the HTTP Archive data it should be easy enough to calculate for the conditions which it tests. The “back-end time” is the “Time to First Byte” (being generous as it will include DNS and socket connect as well) and the full time is the page load time. Time to first byte / page load time = % of time spent on the back end.
To see the actual user experience you need to go to a RUM tool like GA or SOASTA mPulse and see if they have released aggregate stats (or check for just your site).
I’m pretty sure 80/20 is generous (which is why including DNS and socket connect doesn’t worry me much). I’d be surprised if it wasn’t closer to 90/10