I would imagine that tipping is not obligatory. If you're tipping however, it's more convenient and considered a kind of etiquette to offer in notes.
So it depends on the price of the meal. If it's only breakfast I usually tip Rs25 note.
If it's dinner I usually tip 50Rs or Rs100.
If the food was awful, there's no way a penny is coming out of my purse!
That's my personal approach and you won't find this written anywhere as well.
Hope this helps.