Percent deviation from previous value

Hi， mgcchkn 

Take a try formula like below:

Previous Test = 
VAR _PreviousTestDate =
    CALCULATE (
        MAX ( Student[Test Date] ),
        FILTER (
            ALL ( Student ),
            Student[Name] = MAX ( Student[Name] )
                && Student[Test Date] < MAX ( Student[Test Date] )
        )
    )
RETURN
    CALCULATE (
        MAX ( Student[Current Test] ),
        FILTER (
            ALL ( Student ),
            Student[Name] = MAX ( Student[Name] )
                && Student[Test Date] = _PreviousTestDate
        )
    )

Percenatge = DIVIDE(MAX(Student[Current Test])-[Previous Test],[Previous Test])

Best Regards,
Community Support Team _ Eason