Resource Availability report via Excel services

 I want to create a report similar to the resource availability report in 
resource center, i've done assignment work and capacity parts but i cannot calculate availability. İ tried to make calculation (capacity - assignmentwork) results are not correct.
Here is my code;
(SELECT     MSP_EpmResource_UserView.ResourceName, 
MSP_EpmProject_UserView.ProjectName as [RowType], 
DATENAME(month, MSP_EpmAssignmentByDay_UserView.TimeByDay) as [Month],
YEAR(MSP_EpmAssignmentByDay_UserView.TimeByDay)  AS [Year],

                   
ISNULL(SUM(MSP_EpmAssignmentByDay_UserView.AssignmentWork),0) AS [Value]
                     

FROM         MSP_EpmResource_UserView INNER JOIN
                      MSP_EpmTask_UserView INNER JOIN
                      MSP_EpmProject_UserView ON 
MSP_EpmTask_UserView.ProjectUID = MSP_EpmProject_UserView.ProjectUID 
INNER 
JOIN
                      MSP_EpmAssignment ON MSP_EpmTask_UserView.ProjectUID = 
MSP_EpmAssignment.ProjectUID AND
                      MSP_EpmTask_UserView.TaskUID = 
MSP_EpmAssignment.TaskUID ON
                      MSP_EpmResource_UserView.ResourceUID = 
MSP_EpmAssignment.ResourceUID INNER JOIN
                      MSP_EpmAssignmentByDay_UserView ON 
MSP_EpmAssignment.AssignmentUID = 
MSP_EpmAssignmentByDay_UserView.AssignmentUID
WHERE      (MSP_EpmAssignmentByDay_UserView.TimeByDay > CONVERT(DATETIME, DATEADD(month, - 3, CURRENT_TIMESTAMP), 102))               
 AND (MSP_EpmAssignmentByDay_UserView.TimeByDay < CONVERT(DATETIME, DATEADD(month, 10, CURRENT_TIMESTAMP), 102))

GROUP BY MSP_EpmResource_UserView.ResourceName, 
DATENAME(month, MSP_EpmAssignmentByDay_UserView.TimeByDay),
YEAR(MSP_EpmAssignmentByDay_UserView.TimeByDay) ,
                      MSP_EpmProject_UserView.ProjectName)
                      
  UNION


(SELECT     
MSP_EpmResource_UserView.ResourceName, 
'Capacity' as [RowType],
DATENAME(month, MSP_EpmResourceByDay_UserView.TimeByDay) as [Month],  
YEAR(MSP_EpmResourceByDay_UserView.TimeByDay) as [Year], 
             
SUM(MSP_EpmResourceByDay_UserView.Capacity) AS [Value]
FROM         
MSP_EpmResource_UserView 
LEFT OUTER JOIN MSP_EpmResourceByDay_UserView 
ON MSP_EpmResource_UserView.ResourceUID = MSP_EpmResourceByDay_UserView.ResourceUID
WHERE     
(MSP_EpmResourceByDay_UserView.TimeByDay > CONVERT(DATETIME, DATEADD(month, - 3, CURRENT_TIMESTAMP), 102))               
 AND (MSP_EpmResourceByDay_UserView.TimeByDay < CONVERT(DATETIME, DATEADD(month, 10, CURRENT_TIMESTAMP), 102))


GROUP BY MSP_EpmResource_UserView.ResourceName, 
DATENAME(month, MSP_EpmResourceByDay_UserView.TimeByDay),
YEAR(MSP_EpmResourceByDay_UserView.TimeByDay))