"Do this" as in calculate your UAI with your half-yearlies? Absolutely not...
What you can do is to estimate the final HSC mark based on your cumulative RANKS in your subjects at school. The teacher can shed some light as to what HSC mark for that particular subject you can expect with your ranks.
A good teacher will not just look at what your ranked but more importantly your marks in exams and their judgement from your exam responses/assessments and their HSC marking experience (if they have any).