How to calculate the Availability of application or server?

Avalaibility = 100 - 100 * (Unplanned Downtime / Uptime )


Availability % Downtime in Minutes  Downtime per Year   Vendor Jargon
90                     52,560.00              36.5 days                   one nine
99                     5,256.00                 4 days                        two nines
99.9                  525.60                    8.8 hours                    three nines
99.99                52.56                      53 minutes                  four nines
99.999              5.26                        5.3 minutes                 five nines
99.9999            0.53                        32 seconds                 six nines

Document on reverse Proxy

The below link have a good (infact best) document on reverse proxy for dummies
http://www.sans.org/reading_room/whitepapers/webservers/reverse-proxy-proxy-name_302