Hi, there some important instructions that you should know while calculating best of four percentage for Delhi University:

•  You need to include one compulsory language that is English
•  Your second subject should the one in which you wish to seek admission. If you don't include that subject then   2.5% marks will be deducted from your best of four percentage
•  The next two subjects should be academic/elective subjects according to List A. If you don't include subjects as   per List A then 2.5% marks will be deducted from your best of four percentage.

So according to the science subjects offered by TS board, your second Maths subject would be Logic and Unfortunately you cannot add that subject in your best of four calculation as according to List A of DU, Logic isn't considered as an academic/elective subject. If you do add the subject then you would face a 2.5% marks deduction from your aggregate best of four percentage

In order to see the subjects included in the List A and for further information regarding best of four  check the link below 

Https://university.careers360.com/articles/delhi-university-admissions-how-calculate-best-of-four-marks

Good Luck