The merit rating is an internal tool, not intended for students. It does not help you in any way, as you cannot know at what stage in the process they are. Merit ratings can change over a matter of weeks depending on how the individual programme works through their process – e.g. they might initially make a merit rating based only on the grades, and then later update once having reviewed SOPs etc.
So it is of no use for you to know when the merit rating comes out. All you need to know is when the actual admission results come out and that is on 29 March.