And their progression was in lock step during their years in the Tower. Whenever their strength is mentioned in NS it's always referenced as "their strength" implying that they are roughly equal.

1 level of difference has never shown us defference, beyond simple politeness, Sheriam is 2 levels above Beonin and 1 level above the rest of the 6 but there is never deference between them. Second is level 17 but didn't defer much to Bera and Kiruna, even Merana, once she spoke up, wasn't bowled over by women 3-levels higher. But the difference is noticeable with the women 4-levels down at 18 who didn't speak until,spoken to and followed their instructions as though they were orders. But note that these orders were given in a polite way by Kiruna/Bera. In the other hand Citalia flat out bowls Siuan over and expects obedience right from the start, implying a larger gap between her and Siuane than what existed between Kiruna/Bera and Berinicia or Kairen. Thus I theorize 5 levels is the minimum gap or Citalia would have been at least polite to Siaun.