Dynamically calculating an index with a fixed value based on selection criteria

Newbie_1 , Try usinge below DAX

Calculate the EUR/L for the product with the highest market share:

HighestMarketShareEUR_L =
CALCULATE(
MAX('EUR/L Overview'[EUR/L]),
FILTER(
'EUR/L Overview',
'EUR/L Overview'[% Market Share] = MAXX(ALL('EUR/L Overview'), 'EUR/L Overview'[% Market Share])
)
)

Calculate the EUR/L for the product with the top volume rank:
TopVolumeRankEUR_L =
CALCULATE(
MAX('EUR/L Overview'[EUR/L]),
FILTER(
'EUR/L Overview',
RANKX(ALLSELECTED('EUR/L Overview'), CALCULATE(SUM('RSP Overview'[Volumes]))) = 1
)
)

Choose the appropriate EUR/L value based on your criteria (highest market share or top volume rank):
BaseEUR_L =
IF(
[Use Market Share],
[HighestMarketShareEUR_L],
[TopVolumeRankEUR_L]
)

Create the indexation measure:
IndexedEUR_L =
DIVIDE(
'EUR/L Overview'[EUR/L],
[BaseEUR_L]
)

[Use Market Share] is a boolean measure or parameter that you can set to determine whether to use the highest market share or the top volume rank for the base EUR/L value.