At what percentile you are, depends on total number of students and students who have scored lower than you in your board but in case you want to check how the percentile is calculated then you can check below provided formula -

100 x number of candidates in the group wih aggregate marks less than the candidate  / Total number of candidates in the group

E.g :-

If 13711 students are appearing in board exam and 6865 students scored lower than you

then you percentile score for 70% marks in the board = 6865 x 100 /13711 = 50.07%

Good Luck!!!