Working with Multiple Date Time Columns in SQL Server.

hi experts.

I'm looking for help with an SQL query.
I have a SQL table that logs website outages.
It has two datetime columns (Timedown and Timeup)

Example.
ID      timedown                                 timeup                        
--      --------                                  ------
1      22/02/2011 4:25:10 AM            22/02/2011 4:30:30 AM
2      23/02/2011 8:26:11 AM            23/02/2011 8:26:43 AM
3      29/02/2011 10:14:10 PM            29/02/2011 11:30:30 PM

I want to create a SQL query / stored procedure that allows me to calculate the percentage uptime of the website between two selected datetimes. (I.e. between 20/02/2011 1:00 AM and 25/02/2011 5:00 AM, the percentage uptime was 99.98%)

but to get an accurate reading, it needs to calculate the difference between values when an @reportstart or @reportend (or both) parameter is passed that falls not only around the timedown and timeup, but also DURING the outage.

I.e.
timedown       - 2:00 PM,      timeup - 5:00 PM,
reportstart       - 3:00 PM,       reportend 9:00 PM.

so.. the downtime hours during selected report period (3:00 PM to 9:00 PM) is reported as 2 Hours, (not 3).
percentage uptime =  100 - (total hours in selection that website is down = 2 / total hours in selection = 6 * 100 ) = 66.7 % uptime etc

Having trouble constructing a query that can do this.

any ideas?