calculate Monthly average based on YTD value

deevs when working with dates, it is a best practice to add a calendar dimension in your model and you can easily create one following my blog post here Create a basic Date table in your data model for Time Intelligence calculations | PeryTUS IT Solutions

Once you have calendar dimension, create a relationship with the transaction table, it will be one to many relationship.

Add the following two measures

Avg Numerator = 
VAR __current = MIN ( CALCULATE ( MAX ( 'Calendar'[Date] ), ALLSELECTED ( 'Calendar' ), VALUES ( 'Calendar'[Year] ) ), TODAY() ) 
VAR __startOfMonth = EOMONTH ( __current, -1 ) + 1
VAR __endOfMonth = EOMONTH ( __current, 0 )
VAR __daysUntilToday = CALCULATE ( COUNTROWS ( 'Calendar' ), DATESBETWEEN ( 'Calendar'[Date], __startOfMonth, __current ) )
VAR __dayUntilEOM = CALCULATE ( COUNTROWS ( 'Calendar' ), DATESBETWEEN ( 'Calendar'[Date], __startOfMonth, __endOfMonth ) )
VAR __avgNumerator =  MONTH ( __current ) - 1 + ( DIVIDE ( __daysUntilToday, __dayUntilEOM ) )
RETURN __avgNumerator

Average = 
VAR __actual = CALCULATE ( SUM ( 'Table'[Actual] ), ALLSELECTED ( 'Calendar' ), VALUES ( 'Calendar'[Year] ) ) 
RETURN
DIVIDE ( __actual, [Avg Numerator] )

To visualize the data, use the month column from the Calendar table and above measures, and that will do it.