How do I calculate the amount of cash I have available to bid on free agents?
When you are on the auctions screen it tells you how much available cash you have at the top on the left.
You can also bid more than that, and cut players afterwards if it comes to that.
So, under the Finances screen, is available cap space the amount of cash you have to bid on free agents
Without having to cut someone to clear space, yes. This number is also presented on the auction page, per @Wimble