OC,
Wasn't questioning your understanding of the mechanics, but think I am mis-reading where you wrote, "...you are earning a nice return on your deposit. If you lower that then the numbers won't be so good as it makes buying the stock look worse as you opportunity cost is very high." (thought you were suggesting that buying stock looked worse if you lowered the assumed rate earned on deposits.)
I'll take a closer look to understand how the calc. is using the bid and ask per your suggestion. In the end though, I think you will agree that since interest earnings and costs are a function of time, the relative attractiveness of trades in stocks versus futures is a function of the duration of the trade. For example, the benefits of SSF will be considerably larger for someone holding a trade 60 days versus 1 day, and nonexistant for someone day trading. The longer the trade the more likely the difference in interest earned and paid will overcome the larger cost of the spreads in SSF, which is not time dependent.
Thanks again for the post and the link to the calculator.