I think your numbers are O.K.
In theory you need to ferment a further 30 gravity points, so if your fermentation stalled due to lack of nutrients, you would need 10ppm of YAN (Yeast Assimilable Nitrogen) for each further 10 gravity points to be fermented. In practice this is done by adding nutrient such as DAP which is 20% YAN (i.e. 50ppm of DAP per 10 gravity points).
In your case, this is 30/10 x 0.05g DAP x 12 litres = 1.8g of DAP. Your 3.5g of nutrient is probably twice what you really needed, but these numbers can be a bit rubbery so I wouldn't expect such a smalll amount to cause any issues.
I routinely add about 1/4 teaspoon (roughly a gram) of DAP to 5 litres after primary fermentation has finished, just to ensure complete fermentation and don't have any taste/flavour issues. I do this because I often use S04 which is a high nutrient dependent ale yeast and have had ciders stall at around 1.015 if the juice is low in YAN. Juice from late season apples from old, unfertilised trees or juice that has been in cold storage for too long can be low in YAN.
The extra yeast probably wasn't needed as the population would have grown by itself and simply stopped working if it ran out of nutrient (if indeed that was the issue, but it sounds like it).