Since there id difference in the difficulty level of the examination held by tamil nadu board and state board of other states,the percentage is calculated by a normlisation process.For example,if the highest mark obtained by a candidate of Tamil Nadu Board is 100, in say Chemistry, and by a student from another board is 90, both the scores will be treated as equal to 100. If another candidate of the same board scores 60 in the subject (where the highest mark is 90), it will be considered as equivalent to 66.66.
100*60/90 = 66.66

So, to calculate your percentage in a subject you need to know the higest marked scored in that subject in Tamil Nadu board and your own board.Divide the higest mark in TamilNadu board to your board highest and then multipy it with your marks to get your percentage in the particular subject.

If there is any tie between the marking of candidates the rank will be decided in the following manner:
Percentage of marks in Mathematics

Percentage of marks in Physics

Percentage of marks in the fourth optional subject

Date of birth of the candidate (the older candidate will get preference)

Assigned random number (higher the random number, the higher the priority.

For,any more information about this you can refer to the following link:
https://engineering.careers360.com/articles/tnea-rank-list