If you are not interested in the numbers, the only thing you care about is the table that resumes the best way to get gems and maybe the graph underneath. Also, remember to insert the Gem Card and the Gold Seller prices (if you have something different than the spreadsheet).
How to buy in dollars from Arenanet's gem store
Just click on "buy gems" and, as billing country, select something like Algeria. I recommend to pay with paypal.