Why I switched to a 1:8 ratio
For most of last year I brewed at 1:5 and diluted the concentrate with water or milk. It works, but I was never sure how much I was diluting, and the taste drifted from glass to glass.
This summer I ran four jars side by side: 1:4, 1:6, 1:8 and 1:10, same beans (a washed Colombian, medium roast), same grind, same 18 hours in the fridge. I tasted each one straight and with a splash of milk.
The 1:4 and 1:6 jars were syrupy and a little harsh on their own. The 1:10 was pleasant but thin, and disappeared completely in milk. The 1:8 jar was the one I kept going back to: drinkable straight over ice, still present with milk, and the chocolate notes from the bag finally showed up.
What I do now
100 g of coffee, 800 ml of filtered water, coarse grind, stir once, lid on, fridge for 18 hours. No dilution afterwards. It keeps for about a week before it starts tasting flat.
The trade-off is volume: you need more space in the fridge for the same number of drinks. For me that is worth not having to measure again every morning.