Finding a previous value based on another column

PhilipTreacy Thanks for your contribution on this thread.

Hi JL0101 ,

You can update the formula of measure [A-Weight Difference] as below and check if it can return the expected result.

A-Weight Difference =
VAR _Revnum =
    MAX ( 'Full_Rebar'[C-Revision Number] )
VAR PreviousRev =
    CALCULATE (
        MAX ( 'Full_Rebar'[C-Revision Number] ),
        FILTER (
            ALLSELECTED ( 'Full_Rebar' ),
            'Full_Rebar'[C-Revision Number] < _Revnum
        )
    )
VAR _preweight =
    CALCULATE (
        SUM ( 'Full_Rebar'[weight_calculated] ),
        FILTER (
            ALLSELECTED ( 'Full_Rebar' ),
            'Full_Rebar'[C-Revision Number] = PreviousRev
        )
    )
RETURN
    SUM ( 'Full_Rebar'[weight_calculated] ) - _preweight

If the above ones can't help you figure out, please provide some raw data in your table 'Full_Rebar' (exclude sensitive data) with Text format and your expected result with backend logic and special examples. It would be helpful to find out the solution. You can refer the following link to share the required info:

How to provide sample data in the Power BI Forum

And it is better if you can share a simplified pbix file. You can refer the following link to upload the file to the community. Thank you.

How to upload PBI in Community

Best Regards