Hi Praveen,
Sorry. "That sounds about right" -literally it means, as you have read it, "that sounds approximately correct". I used it because I could not be certain that you were absolutely correct, just beacuse I didn't know about bhel poori and pani poori.
In another sense it can mean "That sounds exactly right"
"She lost her keys and locked herself out of the house again today" "That sounds about right"
In other words, we are talking about a very forgetful person, for whom locking herself out of the house is not unusual.
When I said "Praveen, are you sure you're not pulling my leg?" it was because you asked me what an idiom was, but you used a perfect idiom in your question!