Don't top up in your primary. The only point in toping it up is to get rid of the air/oxygen in the fermenter. If it's a primary fermenter and you haven't been opening it 3 times a day, it will be nothing but C02 in it at this point. You don't want the gungk on the sides to get back into your cider. So don't top up until it's in the secondary, and then only do it if you want it to be overly dry. Most of the time, racking it to the secondary will shake loose enough C02 that you won't need to top it there either.