As the 2012 depreciation would have been taken care of in 2012, you need to add the whole 2013 amount.
FC Investment includes topping up yearly depreciation expense, otherwise you are only counting new FC investment each year. Including 2013 depreciation expense ensures you are accruing for depreciation as it goes along, rather than only accounting for large expenses when fixed assets need complete overhaul.
You’re confusing depreciation (expense) with accumulated depreciation.
You don’t get gross PP&E by adding depreciation expense to net PP&E; you get gross PP&E by adding accumulated depreciation to net PP&E.
It might be easier for you to see if you assume some number for accumulated depreciation (A/D) and work from there.
Suppose that 2012 A/D is 200. Then 2012 gross PP&E is 789.9 + 200 = 989.9.
In 2013 we have depreciation expense of 98.5, so 2013 A/D is 200 + 98.5 = 298.5. Net PP&E is 853.7, so gross PP&E is 853.7 + 298.5 = 1,152.2. Thus, FCInv must be 1,152.2 − 989.9 = 162.3.
By the way, you should try it with some other assumed 2012 A/D – say, 300 – to convince yourself that that number doesn’t change the result.
You don’t use the 2012 depreciation expense because it’s already included in the 2012 A/D, so it’s already included in the 2012 net PP&E.