Query Form? LOOKUPVALUE?

Not sure why it isn't working but not being able to see the model I cannot be sure what is going on. That said, you should never use LOOKUPVALUE() except in very special circumstances. It isn't a good function. Performs horribly and maintains an Excel mindset bypassing the model advantages.

Try this measure:

Get Percent =
VAR varDate =
    MAX ( P6Date[Column1] )
VAR Result =
    CALCULATE (
        MAX ( 'Planned P6'[Planned % Value] ),
        KEEPFILTERS ( 'Planned P6'[W/E Date] = varDate )
    )
RETURN
    Result

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