[(Current FMV - Exercise Price)* Number of options to be Exercised] *31.2% (30% + 4% Edu Cess).
Eg : If you want 10 Options to be exercised with exercise price being Re 1 each and Current FMV per option is 1000 then the calculation of Tax is [(1000 - 1) X 10 ] X 31.2% i.e [9,990] X 31.2% = INR 3,117 (perquisite tax).
For NRIs, India and overseas tax implications to be evaluated separately, based on the specific facts. Further, requisite income tax filings (such as Form 15CB / 15CA) to be undertaken.