Need custom Column

Hi tangutoori ,

You can use the code below to create a Calculated Column to get the expect result (please adjust the code as you need):

Max Current =

VAR GroupId = 'Table'[Group ID]

VAR MaxSerialNumber =
CALCULATE(
    MAX('Table'[Serial Number]),
    FILTER('Table', 'Table'[Group ID] = GroupId)
)

RETURN
CALCULATE(
    MAX('Table'[Current]),
    FILTER('Table', 'Table'[Serial Number] = MaxSerialNumber)
)

Hope this answer solves your problem!
If you need any additional help please @ me in your reply.
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Thanks!

Best regards,
Joao Ribeiro