Helli aspirant, 

You should have to score 180 out of 200 to obtain Aplus in asubject in kerala hse board as 90 percentage and above is termed as Aplus.

It can be calculated as follows

200*0.9=180

Similarly for A grade, you have to score 160 out of 200 and so on.

I know you are in your exam preparation which is already started by last week

All the best for your exams.