Do note, if the remaining 40% per project, the one which would take you 60% time to complete, takes 60% time for your co-worker as well - then it's obviously not 40% now, is it?
I think this is the big problem - that's not obviously the case at all. It depends on what the percentages are measuring, and if they're measuring the time the job will take then any sort of 80/20 rule is unlikely to be useful. More likely you're talking about requests in a queue, or items on a feature list, or lines of code in an estimated total required.
The important message to take from "80% of the work takes 20% of the time!" is "once you've got 80% of the work done, budget four times as long to finish the job properly" - important, in my view, because of how often the inverse mistake is made, people looking at the 80% that gets done quickly and easily and not allowing enough time or resources for the fiddlier stuff, or thinking that individuals or teams who do that 80% work on many projects are industrious go-getters who deserve
more resources to let them mostly-do
more projects while the ones trying to actually get those unfinished projects ready for actual
use are shiftless malingerers who take forever over the tiniest tasks.