Woodford,
first off, you need to get your decimal point in the correct place.
1 SSN:ADR(AMEX) = 20 SNN:FPO(ASX)
and, then, account for then exchange rate.
I posted a link yesterday to this spreadsheet, it may help:
http://samsonoilandgas.com/IRM/content/investor_adrprogram.html
--Dan