New Measures, Columns - Data sets.

I think I'm not quite understanding how all this works as I'm stuck again :(  This explanation will be a bit lengthy - sorry, but its complicated what I need - and I cannot provide my data due to confidentiality.

I have multiple tables per month. The data I'm using is a snapshot essentially of about 90K records. Each month, all 90K records have to be pulled. It is based on configuration items in a cmdb. 

They want various metrics based on the CI data - and they want monthly running totals in a total. So a column of December Data, January Data, February Data etc.  I did the first month, where I had to create new measures to do calculations. 

Trying to make a line chart with this data, where the x-axis is the month, and the Y-Axis is either total, or percentage, and the data point for this one chart is "# of config items without relationships"

The challenge is that the Config Item records are a complete list of all of them 90K+. I added a column to the first table "Reporting Month", so that I have a way to differentiate between Data pulled for December, and data pulled for January. 

I created a new measure for the first December dataset to subtract # of Items in one table from the # of items in another set. Worked great.

I then went to do January. Went to create a new measure, and it says a measure already exists. I didn't realize I couldn't create a new measure of the same name for multiple tables!  I can't create a new column, because it isn't a column of data - I just need the one number - and a new number for each month. 

I've seen lots of videos on how to do power bi, but almost all are based on revenue etc - and things are done differently when trying to work out for IPC data. 

What is the best way to handle this? Is there a video someone can point me to that has similar examples? Most of my metric requirements work like this.