Optimizing Data Calculation Inside SP

Hi,
I created a SP which is supposed to calculate some values for me and return them as a resultset. I have a RequestTime field, and a ResponseTime field; This sp should calculate how much time does it take for us to respond to a customer's request. if it is more than a specific time, this sp should calculate the extra time and its fine base on a constant fine-per-extra-minute value.
It should also calculate total fine for all records.
To do so, I wrote it as this:


Code:

CREATE PROCEDURE up_responsetime
@sdate smalldatetime,
@edate smalldatetime,
@TotalFine int OUTPUT
AS
DECLARE @Fine_Per_Min int
DECLARE @Max_ResponseTime int
SET @Fine_Per_Min = 3
SET @Max_ResponseTime = 120

SELECT ID,RequestDate,RequestTime,ResponseDate,ResponseTime,
-- Calculate exceeded amount of time for each record
DATEDIFF(minute,RequestDate+RequestTime,ResponseDate+ResponseTime) - @Max_ResponseTime as ExtraTime,
-- Calculate fine for each record
(DATEDIFF(minute,RequestDate+RequestTime,ResponseDate+ResponseTime) - @Max_ResponseTime) * @Fine_Per_Min as Fine
FROM CusRequests
WHERE RequestDate BETWEEN @sdate AND @edate
--Calculate sum of all fines and return it in TotalFine variable.
SELECT @TotalFine = SUM((DATEDIFF(minute,RequestDate+RequestTime,ResponseDate+ResponseTime) - @Max_ResponseTime) * @Fine_Per_Min) FROM CusRequests
GO



but I have two concerns about this:
1- Calculated Fine field returns a negative figure if the ResponseTime is in the desired period of time. But I want it to return zero for such cases, and return only a positive figure when extra time was spent on responding to the request.

2- As you can see, there are many redundant calculation in this code, and this will affect its performance. I wanna know if there is any more optimized way to write such a code?

I appreciate any help on this.
Thanks alot

p.s. sorry if the post was lengthy.