I am making wine from black homburg grapes and the primary fermentation has completed (10days). I didn't have a hydrometer at the start, but have transferred the juice from the must to 2 1/2 demijohns and noticed there is no more fermentation, so I then found the sg was about 994 and realised I should have added sugar at the start of primary. How can I calculate how much sugar to add at this stage? Should I go back to primary fermentation in an open container with loose cover or keep it in the demijohns?
Any help would be greatly appreciated.