By experience, the Highest 10 or 20 level that you can do,If I am not wrong Bonus lap is based on how high the Floor you are on, eg: Ultimate Floor 41-61 is always Lap Bonus 2
while bellow it is Lap bonus 1, so it will net you more chest every run. But if you want it easy just do the lower floor and its fine as well.
15 Grimoires is crucial for starter, and everything else is nice too, definitely clear everything.