After receiving some feedback from interested parties I've updated the post to show what I'm basing my conversion formula on. I'm using the historical data feed of MtGox transactions provided by
bitcoincharts.com and storing it in this
Google Spreadsheet.
The detail tab in the spreadsheet has the exact data from the bitcoincharts feed. The summary tab shows the daily and 7 day averages for BTC to USD and PLEX from Jita along with my current exchange rate for BTC per PLEX and BTC per Million ISK. I'll be updating this spreadsheet daily with the averages of the prior date.
As a bonus, a multiplier has been added to give you more ISK for each BTC you spend.
Thanks.