Churn Rate Measure

The assumption is that you have a proper Date table in the model that's not connected to your Accounts table (Dates). Once this is in place, you can write:

[# Acc Lost] =
var __periodStart = MIN ( Dates[Date] )
var __periodEnd = MAX ( Dates[Date] )
var __accCount =
	CALCULATE(
		COUNTROWS( Accounts ),
		Accounts[Account Initial Date] < __periodStart,
		Accounts[Account Final Date] >= __periodStart,
		Accounts[Account Final Date] <= __periodEnd
	)
return
	__accCount
	
[# Acc at Beginning] =
var __periodStart = MIN ( Dates[Date] )
var __periodEnd = MAX ( Dates[Date] )
var __accCount =
	CALCULATE(
		COUNTROWS( Accounts ),
		Accounts[Account Initial Date] <= __periodStart,
		OR(
			ISBLANK( Accounts[Account Final Date] ),
			Accounts[Account Final Date] >= __periodStart
		)
	)
return
	__accCount

[Account Churn] = DIVIDE( [# Acc Lost], [# Acc at Beginning] )

You should not slice by [Account Final Date] and [Account Initial Date]. These fields should be hidden.

Best

Darek