Dynamic Variance between Figures

Greg_Deckler thank you 🙂

The below shows you so far - Month_Var has the following DAX: 

FTE_Prev_Mnth = 
CALCULATE(
    [FTE_Demand],
    PARALLELPERIOD(
        Date_Dim[Date],
        -1,
        MONTH)
)

However - how can i make this dynamic, for example not always a period of -1 , say if i selected april-24 and july-24 , id like to see the difference between them two months. 

And also - say if i drill up on the matrix , can Month_Var for example show the difference between the years, all within one measure/column.

Ive tried with the dynamic one - thats just totalling each months FTE total without a filter and comparing to itself, however I thought I was on the correct lines... Seems not!!

Dynamic FTE_Demand Difference = 

var dt =SELECTEDVALUE(Fact_IBP_Data[Cycle_Date])
var min_dt = CALCULATE(MIN(Fact_IBP_Data[Cycle_Date]),Fact_IBP_Data[Cycle_Date]>dt)
var val = CALCULATE(SUM(Fact_IBP_Data[FTE]),Fact_IBP_Data[Cycle_Date]=min_dt) - SUM(Fact_IBP_Data[FTE])
return val