DAX PBI Current month minus Previous month

Question

I have a measure:

_Forecast, % =
VAR ForecastMinimumPercent = 10
VAR ForecastIncreasePercent = 15
VAR sM =
    MAX ( T1[PlanDate] ) //Plan Date
VAR fM =
    MAX ( '_Date2'[Date] ) //fact Date
VAR sC =
    FIRSTNONBLANK ( T1[ProjectCode], 1 ) //Current project code
VAR tP =
    //Current forecast cumulative percent
    CALCULATE (
        SUM ( T1[ForecastPercent] ),
        FILTER (
            ALLSELECTED ( 'T1' ),
            T1[PlanDate] <= fM
                && T1[PlanDate] <= sM
                && T1[ProjectCode] = sC
        ),
        REMOVEFILTERS ( T1[PlanDate] )
    )
VAR mM =
    // Current Project first Date
    CALCULATE (
        MIN ( T1[PlanDate] ),
        FILTER ( ALLSELECTED ( 'T1' ), T1[ProjectCode] = sC ),
        REMOVEFILTERS ( T1[PlanDate] )
    )
VAR nD =
    ROUNDUP ( DIVIDE ( 100 - tP, ForecastIncreasePercent, 0 ), 0 ) // count of forecast columns
VAR fD =
    DATEDIFF ( sM, fM, MONTH ) //dates offsets, '_Date2'[Date] - T1[PlanDate]
VAR P1 = tP + ForecastIncreasePercent * fD //forecasted percent
VAR P2 =
    IF ( P1 > 100, 100, P1 ) //does not show values over 100, for example 90+15=105 but show 100
VAR fP =
    SWITCH (
        TRUE (),
        tP < ForecastMinimumPercent, BLANK (),
        // if percent <10, then hide
        fM < mM, BLANK (),
        //hide values if T1[PlanDate] <> '_Date2'[Date]
        MAX ( T1[ForecastPercent] ) = 0, BLANK (),
        // hide empty percent rows
        fM
            > DATE ( YEAR ( sM ), MONTH ( sM ) + nD + 1, DAY ( sM ) ), BLANK (),
        //does not show 100 more than once
        fM <= sM, IF ( HASONEVALUE ( T1[PlanDate] ), BLANK (), tP ),
        //if collapsed show first fact values
        P2
    )
RETURN
    fP

This measure give me current forecast percent

But i need get difference current month - previous

How to write right syntax?