I see you adjusted to DateDiff(). Yes, that would be the approach to go. I have a similar process and that is the best I have found sofar. The usage of Second() is also a good fit. To handle the decimals, I ended up with something like this:

SELECT COALESCE(SUM(Temp.[Second])/3600.0,0)
 FROM (SELECT DATEDIFF(SECOND,
 CASE WHEN BusinessTime.Start>Client.AddDate THEN BusinessTime.Start ELSE Client.AddDate END,
 CASE WHEN BusinessTime.[End]<Client.FirstContactDate THEN BusinessTime.[End] ELSE Client.FirstContactDate END) AS [Second]
 FROM BusinessTime
 WHERE BusinessTime.Start<Client.AddDate AND
 BusinessTime.[End]>=Client.FirstContactDate) Temp

This is basically and adaption of what it is actually. But, that should give you an idea on the DateDiff() usage with Second, which is then divided by 3600. In my case, I wanted to have that in hours. So, that changed the decimal value into hours, which has decimal support with the ,0.