I've made thousands of slayers and super slayers, my best suggestion on it is to use the first one, as long as you have a 100% success, as u dont need to have the mark on them. the higher your magery mysticism or necro is the better your chances, also it seems like SS eval also help some.
if you dont get one in the first like 3-400 just stop making and try agian later as it seems to run in groups.
some days i can get 5 and osme i get like 30 super slayers it's all a crap shoot, but if u dont get a bunch in the first like 1000 books just stop.