You might not be able to officially call an election on partial counts, but based on the current vote % and the % remaining, you can be reasonably certain what the outcome will be. For instance, if it's 2-1 against (ie. 66% against, 33% for), and 75% of votes have been counted, then it's almost impossible for remaining votes to change the result. It's the basis on how the AP can call elections before all the votes have been counted, and still be mostly correct.