I've bought a chain wear checker made by BBB, but not sure what the percentage of worn is about?

It has 2 sides, one of them indicates that the chain is worn 75% and I should replace it in the near future. The other side shows, that the chain is 100% used and should replace it as soon as possible.

My detailed question :

What does these percentage mean? What is the equivalent of it in the metric system? 

Will 75% worn state affect my cogwheels also? Can I replace the chain to a new one without changing the drivetrain's other elements? 

I use a single-speed bike. Chain width is 1/8 inch. It consists 94 chain. (400km (250miles) / month)


Question: Modal Popup not Closing if multiple Modals are present
Answer: From Main Controller First Open Your Modal1,on Close of Modal1 i.e

 var modalInstance = $modal.open({
                templateUrl: 'abc',
                controller: 'abcCtrl',
                size: 'lg',
                scope: $scope,
             });

            modalInstance.result.then(function(result) {
                  $scope.def();
              }, function(err) {
                //
            });

  $scope.def = function(){
     //open another 
    }


call another function and in that function open another modal and similarly you can do.