DAX Formulas (worng grand totals and missing values)

There is a piece of data (table 1). Primary data starts with March 2010 and I filtered it by Employee 1 and Employee 100.

As you can see Employee 1 was not at the Project in May and December, so in May Total Expenses change was -$700. One part of my task is to show how Total Expenses changes because of structural changes in a team. I created next formulas:

Current Salary:=SUM([Salary])

Salary in Previous Period:=CALCULATE([SalCur]; PREVIOUSMONTH('Calendar'[Date]))

Salary Change:=[SalCur]-[SalPrev]

And everything seems to work fine (table 2) (Grand totals are correct and values for periods when Employee leaves the project are shown) until this moment. Then I write formula, which shows only that values, where Employee comes in or leaves the project. Here it is:

Employee Turnover:=IF(OR([SalCur]-[SalPrev]=-[SalPrev]; [SalPrev]=0); [SalCur]-[SalPrev]; BLANK())

And here is the problem (table 3). If I filter the results by more than 1 employee, grand totals won't sum up correctly, but all values at their place and correclty calculated.

I read on the internet that there is fix for wrong grand totals and I used it.

FIX:=SUMX(VALUES(tbl_Utilization[Employee]); [EmployeeTurnover])

The result (table 4) is that grand totals are correct but values in periods where employee was not at the project are missed. How can I make PowerPivot 'see' negative values? As far as I understand the problem is that there is blank value for Employee 1 salary in May, but can I fix this? Maybe I am using wrong logic to calculate Total Expenses changes because of structural changes if that is the case I am looking forward to the suggestions.

P.S. I am new to Power Pivot

P.S.S I am not native English speaker, so excuse me for grammar mistakes

Hi,
Could you please provide an example file rather than a screen shot? I can not experiment with an image

Regards Kurt

Hi, Kurt!

Here is example file.

That file is merely data with no formulas.

I can't check your logic unless you provide the actually formulas you are using.