On Platform values for font ionicons.woff

Hi

I am using custom font ionicons.woff and have used below;

    <ResourceDictionary>
        <OnPlatform x:TypeArguments="x:String" x:Key="ionicons">
            <On Platform="Android"
              Value="ionicons.woff#Regular" />
            <On Platform="iOS"
              Value="ioniconsr" />
        </OnPlatform>
    </ResourceDictionary>

However this is not working. What are the correct 'On Platform' values for font ionicons.woff and in general what is the method to get these values for a given font.

Thanks

Regards