Hi Chard
It's a good thing you worked it out as well. This problem occured to me once before.
That's Why I created the Strategy Explorer
Not only does it tell you how the credit system works, but also, does it help choosing a strategy.
In fact its 17 rows of a cheap card giving 10 specific credits to a single medium card, which gices 20 specific credits to a single high card
You can find the topic here:
http://www.civproject.net/forum/viewtopic.php?t=478
When you want to calculate things like you created, my cousin created the Civ Administrator. This is even more advanced excel-document using a shopping cart and this is also keeping track of AST positions of all players.
In the end it provide a graphic of all players.
youcan PM him here @ Gerart de Haan