Inventory Availability Coverage

I took the following approach. Note that this answers if you have stock in any warehouse, not all warehouses as you stated, but your results of 25%/75% seemed to indicate it was any warehouse.

1) I created a simple table to get a Yes/No based on stock:

Stock Status = 
SUMMARIZE(
    Inventory,
    Inventory[Product],
    "On Hand",
        VAR IsInStock=
            CALCULATE(
            SUMX(
                Inventory,
                IF(Inventory[Quantity] > 0, 1, 0)
            )
            )
        RETURN
        IF(IsInStock > 0, "Yes","No")
)

It returns this table:

Then I related that table to your inventory data as shown:

Then I created a table visual, dropping in the "On Hand" field from the Stock Status table, and the Percent Available measure from the INventory Table, which is the following measure:

Percent Available = 
VAR ProductCount =
    COUNTROWS(
        DISTINCT( Inventory[Product] )
    )
VAR TotalProductCount =
    COUNTROWS(
        ALL( Inventory[Product] )
    )
RETURN
    DIVIDE(
        ProductCount,
        TotalProductCount,
        0
    )

my PBIX file is here if you want to tinker with it.