I don't really like overly fizzy cider, i would prime with 1 tsp per 1ltr PET. Which should give a slight sparkle rather than a full on fizz. 2 tsp if you want it proper fizzy(and yuk-my opinion)
To be honest I am not a huge cider fan, I like it in summer fizzy and cold as a thirst quencher. My wife will love it though no matter how it turns out.