Calculating UK National Insurance rates

Hi Molin 
I had the chance today to look into your file and write some code. I will send you the file in a private message. Following is the description of the solution. However, I still have doubts on whether to consider the calculations on weekly basis or accomulated monthly bases. At the end I decided to go with the weekly based calculation.
The first step was to import the tax lookup table 

Then created the relationship.

Created the measures

Actual Cost = SUM ( LaborCost[ActualCost] )

NI Measure = 
SUMX ( 
    CROSSJOIN ( VALUES ( DateTable[YYYYWW] ), VALUES ( LaborCost[PayrollId] ) ),
    VAR Cost = [Actual Cost]
    VAR R1 = CALCULATE ( VALUES ( Tax[R1] ), CROSSFILTER ( LaborCost[NICategory], Tax[Category], BOTH ) )
    VAR R2 = CALCULATE ( VALUES ( Tax[R2] ), CROSSFILTER ( LaborCost[NICategory], Tax[Category], BOTH ) )
    VAR R3 = CALCULATE ( VALUES ( Tax[R3] ), CROSSFILTER ( LaborCost[NICategory], Tax[Category], BOTH ) )
    RETURN
    SWITCH(
        TRUE(),
        Cost < 120, 0,
        Cost >= 120 && Cost <= 184, ( Cost - 120 ) * R1,
        Cost > 184 && Cost <= 967, 64 * R1 + ( Cost - 184 ) * R2,
        64 * R1 + 783 * R2 + ( Cost - 967 ) * R3 
    )
)

This is how the report looks like