I hope you are well,
I wish on PowerBI when all the months are selected to have the same values for Index, Cross Multiplication -1 and Cross Multiplication -2
(see image below).

Then as soon as I go to the month of April for example Cross multiplication -2 recalculates the index to base 100 from the month of April and so on.
(see image below)

The reason you were getting wrong results for cross multiplication was that you were removing all filters from Base table. In this way, you were removing filter from elements column as well, so it was giving you wrong result. I just made a small tweak in the formula and it is working fine. Please find below formula and attached PBIX.

Cross Multiplication - 2 =
VAR _Selected_Value =
SUM( Base[values] )
VAR _Min_Date =
CALCULATE( MIN( Base[month]) ,
ALLSELECTED( Base ) )
VAR _First_Occurence_Value_For_Each_Category =
CALCULATE( SUM( Base[values] ) ,
FILTER( ALLEXCEPT( Base ,Base[elements]) ,
Base[month] = _Min_Date ) )
VAR _Results =
DIVIDE(
_Selected_Value * 100 ,
_First_Occurence_Value_For_Each_Category ,
0 )
RETURN
_Results