Fill in value 2

Value 2 is what you want to return when the condition is false. In this case, our condition is false when the month is January or February. In this case, we want to return the February value divided by 2. That way we are distributing the February value between January and February. That expression is simply  


Your final formula should look like this: 

If(month()>2, momentum(cntrad1290, 1), cntrad1290/2)

To see full effect please set 'Observations' to 'All points' and 'Conversion settings' to 'Next value':