Red Grape Concentrate?

[Dugger]

For the Blackberry wine under the Recipe section, addition of Red Grape concentrate is listed, but no quantity - typically how much of this is used? They come in 500 ml and one litre bottles here - I added 250 mls (one cup) to a 3 gal batch, but just guessed.

 

[winemaker_3352]

I used it in my choke cherry wine - i added 300 ml to one gallon.