Hello brewers; I need some advice on how to get my target of 5 litres. The last two batches I've done have always been too short (the latest I thought I could add to the boil half a litre to make it up but ended up diluting my beer so it never reached it's OG and FG) How much water do I need...

Hi guys and Ladies; I'm doing a second batch of my Jamestown Porter and I want to add frozen blackberries to cool the wort down and add to taste. Now hows best to do this? I need to add i'm batch brewing 5 litres. THANKS!!!