Dude, me sorry, don't have the song, don't play this style of music, and the last time I heard it it was a long time ago. Maybe 20 years ago. So sorry for missing your post cause I don't hang around here much.
The way I find chords is to listen to the song and hear the chords. From the tab it looks like the scale is (next time transcribe the notes please, it's difficult to do this with no guitar around in office)
F# G# A
And there's a an A# in there in the chorus line as well, but I suspect that's a leading tone.
Also note that the Indian scale system is different from the western scale system, so the chords will be approximately correct, not exactly.
My money is on F#m as it's the last note in the chorus line, and the flattened third looks like the beginning of a minor scale. If it is in minor, else A major would work - but the A# makes me think it doesn't.
And singing to myself (this is weird), it sounds like the song is in 3/4.
Methodolgy: None, in these cases no amount of theory can help. You have to listen and play and figure out the notes by ear. No amount of rules and theory can help before you figure the song out. After that you can theorize all you like.
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