I'm not an expert as I'm only on my 4th batch of cider. All my research says your to use 3/4 cup sugar "I use corn sugar" per 5 gallons. So, if you do the math for 1 gallon you should use around 2 1/2 Tablespoons of sugar for the gallon. I just primed and bottled 2.5 gallons today using this rule, I cant tell you how it turned out yet....its only been 5 hours in the bottle.
As long as the fermentation is done and its not still bubbling away....you shouldn't have any bombs. a hydrometer reading could help with this, but I've had good luck letting it sit for several days maybe a week after no signs of bubbling and you should be fine.