Best Approach to Two Dax Challenges?

Hi,

Let's solve the first one.  Try this

=(CALCULATE(SUM(ConsumptionData2017[Gal (000s)]),ConsumptionData2017[Rate Class]="Residential")/CALCULATE(DISTINCTCOUNT(ConsumptionData2017[Location ID]),ConsumptionData2017[Rate Class]="Residential"))/12

Does this work?

Looks like you're missing a DIVIDE() statement in the first parameter of your CALCULATE() ...

Test Dax :=
CALCULATE (
    DIVIDE (
        AVERAGE ( ConsumptionData2017[Gal (000s)] ),
        DISTINCTCOUNT ( ConsumptionData2017[Location ID] )
    ),
    ConsumptionData2017[Rate Class] = "Residential"
)

PS...go to www.daxformatter.com to get the nicely formatted code that I've been providing.  It's a HUGE help, especially with more complex DAX.

Regarding wanting to see the monthly average in the grand total, you may want to use the following pattern:

IF(

  HASONEVALUE( Month ),

  [Subtotal measure for 1 month],

  [Different measure for calculating in the grand total]

) 

There's some good documentation on this already.  Check PowerPivotPro's website.