OK, here's some simple maths. Let's say a 500M barrel find is...

OK, here's some simple maths.
Let's say a 500M barrel find is confirmed, and the market value that at $10 AUD a barrel. So a $5B find.
Cairn have about 575M shares on issue (got that from yahoo finance by dividing MC by current SP)
The pound is worth about $1.85.
So, to CNE the find is worth 60% * AUD5B / 1.85 AUD/GBP = about GBP 1.6B. Or about 280p per share. Or about 150% gain.
To FAR, a $5B find is worth 15% * $5B = $750M or about 28cps, or about 350% gain.
Incrementally, every 1% gain in CNE attributable to the FAN-1 translates to about a 2.3% gain in FAR.
However, how a find affects future prospects (e.g. a takeover for FAR, ability for CNE to pay India liabilities etc) and how the market values that is anybody's guess.
Incidentally, for ConocoPhilips, 15% of an AUD5B find would add far less than 1% to their MC.