Hi ,

As it is compulsory to pass in all subjects in CBSE 12th board excluding additional subject , and if you took additional subject (biology) and don't want to give additional exam. So don't worry it will not affect on your overall result, as it is additional subject it depends on you whether you want to give additional's exam or not .

So  if you don't give additional subject exam there will be no bad impact on your overall percentage.

Hope!! This will help you .

If you are able to manage your main subject that can only be mathematics and biology at the same time then OK

Otherwise It will be difficult sometimes to manage both subject in boards

This can decrease your overall percentage