If you're using Google Analytics (or most other analytics packages), you don't have to calculate the bounce rate yourself, as the analytics software will do it for you.
If you want to calculate by hand, the formula is:
The number of people who viewed a page on the site and left without visiting another page in the site, divided by the total number of visits to the page.
For instance: if a page had 150 total visits, and of those, 90 visits landed on the page then left the site without continuing on to another page, the bounce rate would be 90/150, or 60%.
Bounce rate is fairly meaningless at the site level. It's much more informative and useful when calculated at the page level. At the page level, it allows you to see which pages are less attractive to site visitors -- these will generally be the pages with the highest bounce rates. These are the pages you need to spend the most time on. Analyze what is driving visitors away from those pages, and fix it.
Where it gets really
interesting and useful is when you segment your traffic by source and figure the bounce rate on a per page basis by source. That way, you can compare the bounce rates among various advertising campaigns, organic search traffic, referral traffic (even down to the referring domain level, if you want to get that detailed about it). If you see a particular segment that bounces significantly more (or less) than the rest, that becomes something to investigate, and could perhaps drive changes in your marketing tactics or budget allocation.
In your example, the bounce rate for the landing page would be zero, because all the visitors continued on to another page on the site.
The interesting part would be to look at the bounce rates of those inner pages for the visitors who arrived from the landing page. For instance, if every visitor proceeds from the landing page to an inner page of the site, but then 100% of them bounce from the inner page, you know your landing page is doing a good job of intriguing the visitors, but the inner pages are letting you down and need some work.