My suggestion is to reverse engineer the process. Start out by thinking what kind of wine you want. Do you want a big, monster wine that will age well for years? On the other hand, do you want something refreshing you can drink quickly? Do you want it sweet or dry? With those ingredients, I think of summer in the south, and I’d want something refreshing. I translate this into a fairly low starting gravity with some residual sugar left over in the end for a slight sweetness and a little fruitiness in the nose.
With that in mind, I’d shoot for an original gravity of about 1.1000. This isn’t very hard to figure out with honey. You can figure about 95% attenuation. I consistently hit near that gravity using 3 pounds of honey per gallon. The berries are another problem. You have no idea of the sugar content of the juice, or how much of the berry is actually juice. I’d press them and get a known quantity of juice and go from there. Once I had my juice, I’d then make a very small batch of must with the ratio I wanted. Let’s say you decide on 4 parts juice to 1 part honey. Then pick a measuring unit you like and blend up a mini batch. Since I work in US gallons, I’d use cups as my measure of choice for this. So, take a large mixing bowl and dump in 4 cups of juice and 1 cup of honey. Mix it well. The gravity of such a mix will be very high…maybe beyond the capacity of your hydrometer. Add a carefully measured quantity of water until you can get a gravity reading. Just guessing, I’d go with 2 cups for the first shot. Then keep adding water until you get a must with a gravity of 1.100 or whatever target you choose. Then you can just scale up your recipe based on the scarcest ingredient.
For example, let’s say you have five gallons of juice, 2 gallons of honey, and you’ve found that you needed to add 3 parts (cups) water to get your mine must to the target gravity. So you have 100 cups of juice and 40 cups of honey. With this scenario, your scarcest ingredient is juice, since you need 4 times as much juice as you do honey. So take your 100 cups of juice and divide by four, which gives you 25 cups of honey. From this, we get the basis of a recipe: 5 gallons of juice and 1.6 gallons of honey. Now all we need to do is figure out how much water to add. Since water is three parts and juice is four, we can just figure out what three-fourths of five gallons is: 3.75 gallons of water. Add all that up and we get 5 + 1.6 + 3.75 = 10.35 gallons of must. You could easily scale this back to a 6-gallon fermenter by dividing all the quantities by 2.
Caveat: The above post is likely the product of homebrew fueled wild speculation and most likely has no basis in fact.