Help with a measure

Hello here is my problem, i am trying to return a particular code through a measure from the table below.

at the moment code below doesn't contains "Issue Purpose" in it so it returns first value it finds. Can someone help

Measure Test =
VAR CurrentRank = SELECTEDVALUE( Forecast[Max rank] )
VAR IssuePurposeCount = DISTINCTCOUNT ( Forecast[IssuePurpose] )
RETURN
IF (
    NOT ISBLANK ( CurrentRank ),
    IF (
        CurrentRank <= IssuePurposeCount,
        LASTNONBLANK (
            TOPN ( IssuePurposeCount, VALUES ( Forecast[Milestone Codes]),Forecast[Milestone Codes],ASC ),
            0
        )
    ),
Blank()
)

Issue Purpose Description Index
FYI     For Your Information    1
IAB    Issued For As-Built    3
ICH    Issued for Construction with Holds    7
ICR     Issued For Construction Record    4
IFA     Issued For Approval    5
IFB     Issued For Bid    6