Month over Month Calculation within Scripts

[CyberCannon]

Hello Everyone,

Hope you all are doing well and are getting ready for the holiday's!  I've come across an issue that I have been trying to program for the past day or two, and just cannot wrap my head around it.  Basically, I am trying to create a month-over-month and year-over-year pivot table, but do not want to use my existing (Below) function due to this not being 100% accurate.

I've tried using Lookup and conditional Sums - hopefully it is just a syntax error.  My table looks like this:

Month	Previous Month	Quantity (=sum(DailyQuantity)	MoM (WANT TO ADD IN TABLE)
Dec 2021	Nov 2021	55	189%
Nov 2021	Oct 2021	29	64%
Oct 2021	Sep 2921	45	-

To calculate Month-over-Month, I want to use the Quantity of Dec 2021 and divide by the Quantity of Nov 2021 (55/29=189%)

I have tried a few set expressions: 

1. =sum({<'Month'={'=addmonths([Month],-1)'}>}[Quantity])
   1. Results the same quantity as original month
2. =Sum({$<[Month]={"$(=[Previous Month]"},[Previous Month]=>}[Quantity])
   1. Returns null

Not sure where to go next.  Unfortunately using the "Above" and "Below" functions just won't work in our use case.  Do y'all have any other ideas that I can try looking at?

Thanks and have a great day!

-Cannon