Well if its about Inflation adjustment only it will be around £7mn in UK (Current Highest Dhoom 3 £2.85mn), $6mn Approx in USA (Current Highest Dangal $12.5mn) and around $3.5-4mn in Rest of World. This give us around $19-20mn in same set of market.
But why is Kabhi Khushi Kabhi Gham legendary goes to the fact that Overseas market then was a very limited with Release size upto 150 Prints only. Since then Gulf, Pakistan & Australia are 3 major markets with Potential of $17-20mn which was hardly $1mn back then.
So when you give such titles like Biggest Hit in Overseas ever its considering Growth factor in Business not by simply Inflation. Sort of Impact K3G had then is equivalent to $50mn Approx in Main Markets or even Higher.