Completion Percentage Rates

I wish that students did not have to get a 100% on an assignment to receive all the percentage points for their completion of the course total.  Since students need to get that 100% as soon as they miss one they just put in random answers so that they can start again.  It wastes a lot of time plus causes a lot of frustration on the student end.  I think that as long as 80% or higher is achieved that the full percentage points for completion should be earned.