To calculate your TNEA score, you should know about the top score of each of these subjects in your respective board. 

Let the top score in Maths, Physics, and Chemistry be M, P, and C, respectively.

Let your score in Maths, Physics, and Chemistry be m, p, and c, respectively.

Your score out of 200 will be then calculated as per the following rule:

Score = [ m/M + 0.5(p/P + c/C) ] x 100

You can use the below link to get an idea about the expected colleges which could be allotted to you:

https://engineering.careers360.com/tnea-college-predictor

Hope this helps!