I would like to validate your logic.
In Dec you compute 120M minus the November value, but in Jan you compute the resulting value minus the January value. So in one case it's the previous value minus the previous month and in the other case it's the previous value minus the current month.
In the attached, I used the lookup function along with the Previous_value function to get the numbers if you need to substract the previous month.
if first() = 0 then [Constant]
else PREVIOUS_VALUE(0) - LOOKUP( SUM([Sum]),-1 )
It may need some adjustment depending on what exactly you want.
Book1samplez_v10.5.twbx 79.8 KB
Thank you so much Michel! Exactly what I needed.