It was stated by devs in past. Once you get girl(from pachinko 900 roll), you roll for what girl. x1 for troll girls, x1.5 for pachinko only girls and x3 for current event girls.

So unless you want to waste your kobans, farm troll girls to get all of them (not that hard, took me less than 1,5 year to get all), then proceed to roll pachinko. Unless you want specific girl, just burn all your saved kobans to get as many during any time period. In case you get all pachinko-only (doubtful, given retarded price [that was then multipled by 10]), girls, wait till next event and then roll.