How to "flatten" a variable into constant?

Hello All,

I have 2 curves that contains a low and high case sales scenario. I would like to find the ratio between these 2 curves (Sales High / Sales Low) on the date of the latest monthly actual sales. I then want to use this ratio as a constant to multiply Sales Low over a range of dates.

Test_Measure =
VAR _eval_date =
    CALCULATE ( MAXX ( ALLSELECTED ( Actual_Sales ), [date] ) ) // Finds latest sales date 
VAR _date =
    // If latest sales date is older than 2 months, it will default back to current month.  
    CALCULATE (
        IF (
            _eval_date
                < DATE ( YEAR ( TODAY () ), MONTH ( TODAY () ) - 1, 1 ),
            DATE ( YEAR ( TODAY () ), MONTH ( TODAY () ), 1 ),
            _eval_date
        )
    )
VAR _sales_high =
    CALCULATE (
        // Value of sales high curve @ _date
        MAX ( Sales_High[sales] ),
        FILTER ( Sales_High, Sales_High[date] = _date )
    )
VAR _sales_low =
    CALCULATE (
        // Value of sales low curve @ _date
        MAX ( Sales_Low[sales] ),
        FILTER ( Sales_Low, Sales_Low[date] = _date )
    )
VAR _ratio =
    DIVIDE ( _sales_low, _sales_high ) // Ratio of Sales Low/Sales High
RETURN
    IF (
        MIN ( Sales_Low[date] ) >= _date,
        CALCULATE ( SUM ( Sales_Low[sales] ) * _ratio )
    )
// Multiply the Sales Low curve by _ratio for all future dates

I've included a sample file below:

Sample File Link

Lets say I get _ratio = 1.57

I would expect to get "Projected Sales" in the first figure as my final result. Instead I get the below figure, where I only have a single value. The curve in the first figure was made by replacing "SUM(Sales_Low[sales]) * _ratio" with "SUM(Sales_Low[sales]) * 1.57" meaning _ratio isnt actually a constant. I assume it has to do with the "IF Sales_Low[date] >= _date" condition.