LOD Calc - Is it right???

Hello all,

I am still trying to get my head wrapped around this idea of an LOD calc and I need someone to double check my work.

Here is my case use scenario:

I am trying to visualized the costs associated with my company's wireless program. The question at hand is "What is the average cost for a device type (i.e. iPhone, Tablet) for the last 6 months per organization. In other words, what I am trying to do is to compare the last 6 months costs for a device from Org A to Org B to Org C.

Let's say Org A is paying an average of $48 a month for an iPhone and Org B is paying an average of $59 a month for a iPhone, I can use this knowledge to help negotiate a better rate with the carrier.

The LOD I am currently using is this:

{FIXED [Device ID]:SUM(IF [Month of Service]>=DATEADD('month',-6,{max([Month of Service])}) THEN [Monthly Charge] END)} / 6

What is throwing me off is that a the Device ID level which is the lowest granularity, there is two charges for one months:

Jan 2019 - SP-352009064586279 $15

Jan 2019 - SP-352009064586279 $25

So the total charge here is $40 and the average cost should be $40 for this example. Tableau gives me a average cost of $20 as there are two rows in this example. I have tried an Exclude LOD as well but that doesn't seem to work for me as well.

Ultimately what I am trying to show is a bar graph that show the last 6 months average cost per device type per org. Beside it I would show the monthly costs on a line graph per org, like this:

LOD Calc - Is it right???