Ok, so I've done a couple of quick calculations, and I can't make much sense of the recipe either!
Considering just the juice part:
I've no idea what the minimum Brix value for naatje juice from concentrate is in South Africa, so I'm treating it as mandarin and assuming this is 11.2.
Your 65 Brix therefore has a w/w concentration factory of 65/11.2 = 5.8.
1000kg of concentrate is therefore equivalent to 1000*5.8 = 5800kg of single-strength juice.
If the juice content of your finished recipe, once diluted, is supposed to be 6%, this means that your 1000kg of concentrate makes 5800*(100/6) = 96726.2kg of finished squash at drinking strength. This seems like a very high value, for a recipe that uses less than 4000kg of ingredients and is supposed to be diluted 1:3!
Considering the whole recipe:
As a very rough indicative starting point, I'd estimate the Brix of the recipe before the 1:3 dilution as: 65*(1000/3847) + 67*(1800/3847) = 48.2 Brix. This ignores any Brix contributions from anything other than the naatje and syrup. Diluting this 1:3 by weight gives you 48.2/4 = 12.05 Brix, which doesn't match your target at all.
Using another rough calculation based on the Brix, I estimate the density of the recipe as 1.2187g/ml. We can then estimate a final Brix for a 1:3 dilution by volume as (48.2*1.2187)/4 = 14.7 Brix. This still doesn't match up with your expected values.
As a final thought, the closest I managed to get to the correct % juice figure was by assuming the Naatje was a single-strength juice rather than a concentrate: 1000kg of juice in the 3847kg mix, subsequently diluted 1+3, gives 1000/(3847*4) = 6.5%, so almost correct. The Brix figures still don't make sense though.
Is this a recipe you've been given by a customer to match? I've "enjoyed" many similar challenges where the customer doesn't actually appear to know anything about their product, and making sense of it is extremely difficult. I think all you can potentially do in this situation is present the result of following exactly what the BoM says, complete with "wrong" Brix values and very high juice content compared to target, and ask if that is what they actually want.
