Create calculated row

try this

EDIT - removed inactive link, uploaded the sample file

assuming there is Tax% in classification table
you create a new measure defining the new ratio

RatioTax :=
DIVIDE (
    CALCULATE (
        [RegularSum],
        ALL ( 'Classification'[Classification] ),
        'Classification'[Classification] = "Trading Income"
    ),
    CALCULATE (
        [RegularSum],
        ALL ( 'Classification'[Classification] ),
        'Classification'[Classification] = "Taxation"
    )
)

and adjust the final measure

SumAndRatio:=
VAR varClassification = UPPER(IF(HASONEVALUE(Classification[Classification]),VALUES(Classification[Classification]),BLANK()))
RETURN
SWITCH(varClassification,
	"COS%",[RatioCoS],
	"Tax%",[RatioTax],
	[RegularSum]
)

etc.

you could also consider extending the classification table with the definions of ratios, and use that as more general pattern, e.g.

Ratio = 
VAR Nom = SELECTEDVALUE(Classification[Nominator])
VAR Denom = SELECTEDVALUE(Classification[Denominator])
RETURN
DIVIDE (
    CALCULATE (
        [RegularSum],
        ALL ( 'Classification'[Classification] ),
        'Classification'[Classification] = Nom
    ),
    CALCULATE (
        [RegularSum],
        ALL ( 'Classification'[Classification] ),
        'Classification'[Classification] = Denom
    )
)

SumAndRatio = 
VAR varRatioFlag = IF(HASONEVALUE(Classification[RatioFlag]),VALUES(Classification[RatioFlag]),BLANK())
RETURN
IF(varRatioFlag,[Ratio],[RegularSum])