Say there are 300 places in a randomly selected course from the two given. This year, 299 people with 99+ UAIs apply to it. The 300th person to apply has a UAI of 52.36.
The cut off would be 52.36.
It's a system within which it is pretty much impossible to predict what the 'cut off' for any given course in any given year will be, unless they are set in advance to a fixed value by the university itself (for example, combined law at Wollongong, which is always 91.00). Your best bet is to look in the UAC guide which you should have a copy of (or failing that, the website), which will show you the trend for the last two years or so.