This is a balancing act. You want a good balance between the salty tastes of the fish sauce, soya and the sour tastes of the vinegar and lime juice. Be careful with the sugar its for balancing the salt and sour.
You can if you like add a bit of chili but I think this is much better without.
Don't throw away your old oyster shells you can reuse them for this. You can also purchase in your local hobby shop bags of small shells that you can fill, wash and reuse.