Calculation Showing Inaccurate Answer
You should get the correct result if you perform your calculation using the sums for each part instead of performing the calculation on each row and summing the result. 

rextester: http://rextester.com/ZGQ51639

Declare @Calc Table
(
  employee varchar(100)
  ,annualsales decimal(10,2)
  ,dep1sales decimal(10,2)
  ,dep2sales decimal(10,2)
  ,dep3sales decimal(10,2)
  ,dep4sales decimal(10,2)
  ,dep5sales decimal(10,2)
  ,tax decimal(10,2)
)


Insert Into @Calc (employee, annualsales, dep1sales, dep2sales, dep3sales, dep4sales, dep5sales, tax) Values
('emp1', '1111.00', '26.12', '83.00', '16.55', '21.77', '87.08', null)
,('emp1', null, '28.07', '11.00', '101.24', '47.03', '76.19', null)
,('emp1', null, '35.25', '7.00', '33.64', '43.54', '43.52', null)
,('emp2', 1954.85, '1120.65', '0', '0', '0', '0', 392.70)


Select 
employee
,AnnualSales=sum(isnull(annualsales,0)) 
,DeptSales  =sum(isnull(dep1sales,0)+isnull(dep2sales,0)+isnull(dep3sales,0)+isnull(dep4sales,0)+isnull(dep5sales,0)) 
,Tax        =sum(isnull(tax,0)) 
,[SomeCalc] =(sum(isnull(annualsales,0))
             -(
               sum(isnull(dep1sales,0)+isnull(dep2sales,0)+isnull(dep3sales,0)+isnull(dep4sales,0)+isnull(dep5sales,0)) 
             +(sum(isnull(tax,0))*.75)
              )
             )
            /nullif(sum(isnull(annualsales,0)),0)
from @Calc
group by employee
order by employee asc