Its based relatively on how well your class went in the externals. So really you can't know that at the moment, but in theory if you put in a constant effort all year then you should have a similar assessment mark. In practice, you can have students ranked below you doing better than you in the externals and students ranked above you doing better than you in the externals and hence you could get a higher assessment mark than your examination mark. Again this is all relative to your rank, difference between ranks and the performance of your class as a whole in the externals.