I'm still curious though as to why the earlier example is not correct... ie what is the result you expect to see ?

If you changed time to 00:35 the formula would return 100% ... ie they completed 5 items which equates to 100% of the items expected.

If you subsequently revert time worked to 07:30 you get 7.78% ... ie they completed 5 items out of the expected 64.28571 items - aka 7.78%