What-If Parameter

Hi Plump_Lil_Monk ,

Try following logic:
1. Create your What-If parameter in Model section:

2.  Create a new measure. Measure should work only for IT and Communication, for the rest the hours will not change. You can try something like this:

Editable Hours =

var _Activity = SELECTEDVALUE(TableX[Booking Activities])

var _Hours = SUM(TableX[Hours Booked])

return

    SWITCH(

    _Activity,

    "Communication",_Hours * Parameter[Parameter Value],

    "IT",_Hours * Parameter[Parameter Value],

    _Hours)

FYI - this measure works also with Parameter Value, which is value connected to your What If table and slicer. Combining Parameter Value with Hours will solve your problem.

I Have an example in my own matrix.
As you can see, slicer has value = 10, so my hours in IT and Communication are 10x bigger. Other values are the same: