There is some info missing that would allow you to calculate these things.
For the first step, the concentrate is 8.2mg/100g. To work out the single-strength juice content of this, you'd need to know how concentrated the blackcurrant is. Typically this would be commercially produced as a 65 Brix product, so from that assumption you can to an extent estimate the juice content.
Normally the target Brix for this type of product uses uncorrected refractometric Brix, so you can estimate the corrected Brix value by also estimating the acidity - it's a relatively acidic fruit, so I don't think you'd go too far wrong assuming that the titratable acid content is at least 15% weight/weight as anhydrous citric acid.
From the acidity you can find the correction to the Brix value using e.g. the USCL Customs Methods document I shared in this thread (which is probably worth a read through in general on this topic): https://www.ifsqn.co...ted-bx-content/
From that, you can see that the correction is +2.81 Brix, so we will assume that the Brix of the concentrate is 65+2.81 = 67.81.
We then need the Brix value of Blackcurrant at single strength. This may depend on where the product is being sold, as e.g. the US has slightly different minimum Brix values to the UK/EU for some fruits. In the EU, the minimum for blackcurrant would be 11.6.
We can then estimate the concentration factor as the concentrate Brix divided by the single strength Brix, i.e. 67.81 / 11.6 = 5.846.
This means that 1g of concentrate is equivalent to 5.846g of juice, so 8.2mg concentrate is equivalent to 8.2*5.846 = 47.937mg of juice.
For the next part of the calculation, you'd need to know (a) what the dilution rate is, and (b) what the serving size is.