Sample DNA 2 (Source)

Quote: The highest CL a MBE can sample is 75. The only creature of this level is Mutant Rancor.

This is an approximation of the highest CL at each skill level

Novice BE -> CL 14 (i updated from 12 in accordance to user reply)
DNA I -> CL 28 (i updated from 23 accordance to user reply)
DNA II -> CL 34 (possibly low to)
DNA III -> CL 45(possibly low to)
DNA IV -> CL 56 (possibly low to)
MBE -> CL 75 (appears to be the cap)

Note: Creatures do a detection check after you sample. This detection check is similar to the one used for mask scent with ? and ! representing mobs being alerted to your presence and then finaly spotting you