Divide and Filter by Row condition

I am not sure the requirement as it is little confusing. I would like provide below details, so that it help you resolve.

Power Query:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("bY67DcAwCAVXiagp8rDAafNdAnn/NWI5KWIcyuP0dO60EtPUHViTBKrZqLDT1nMBm1p0Z23uPiwLwzTQ9C4fPx2mCBTybJ9DhyJ21MtLs6/vB1aHkUa5llMpNw==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Letters = _t, #"Volume A" = _t, #"Volume B" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Letters", type text}, {"Volume A", Int64.Type}, {"Volume B", Int64.Type}})
in
    #"Changed Type"

Add all measures using Dax:

Divide Measure = DIVIDE([Sum B], [Sum A], 0.00)

Divide Measure 2 = DIVIDE([SUM B GT 500], [SUM A GT 500], 0.00)

Is GT 500 = IF ( HASONEVALUE('Table GT 500'[Letters]), If ( [Sum B] > 500, 1, 0), BLANK())

Sum A = Sum('Table GT 500'[Volume A])

SUM A GT 500 = 
   -- only for those [Sum B] gt 500
   IF ( HASONEVALUE('Table GT 500'[Letters]), If ( [Sum B] > 500, [Sum A], BLANK()), 
             CALCULATE([Sum A], Filter('Table GT 500', [Sum B] > 500))
    )

Sum B = Sum('Table GT 500'[Volume b])

SUM B GT 500 = 
    IF ( HASONEVALUE('Table GT 500'[Letters]), If ( [Sum B] > 500, [Sum B], BLANK()), 
            CALCULATE([Sum B], Filter('Table GT 500', [Sum B] > 500))
    )

Set the both divide measures format as percentage

Output:

Now, you can refine to your needs! Mark it as answered if it helps you. (and Kudos)

Hi ING_BT ,

Please try:

Measure = 
IF(
    HASONEVALUE('Table'[Letters]),
    DIVIDE([Sum of Volume B],[Sum of Volume A]),
    VAR _B = 
    SUMX(
        FILTER(
            SUMMARIZE(
                'Table',
                'Table'[Letters],
                "B",[Sum of Volume B]
            ),
            [B]>500
        ),
        [B]
    )
    VAR _A =
    SUMX(
        FILTER(
            SUMMARIZE(
                'Table',
                'Table'[Letters],
                "A",[Sum of Volume A],
                "B",[Sum of Volume B]
            ),
            [B]>500
        ),
        [A]
    )
    RETURN
    DIVIDE(_B,_A)
)

Best Regards,
Gao
Community Support Team

If there is any post helps, then please consider Accept it as the solution  to help the other members find it more quickly. If I misunderstand your needs or you still have problems on it, please feel free to let us know. Thanks a lot!

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Hello ! 

Thank you both for your help ! 


But, I think this doesn't quite apply as here it considers that Volume A and Volume B are in the same table which is not the case: 

TableA[VolumeA] + Column LettersA
TableB[VolumeB] + Column Letters B
TableC[Letters]

Table C connects to TableA and TableB via "Letters" in a One to Many relationship on both. 

I've tried to adapt the measure - 

Thank you again! 

Best,
BT