That's actually something I am working on getting a table for, since the rank table shows position (lance leader, company commander, etc.) not rank (Sergent, Lieutenant, etc.). I'll post those when I'm done with them, but I am guessing for ComStar and the Word of Blake it's something like this:
Level I -Acolyte: Rank 3
Level II: Adept Officer Rank 4
Level III: Demi_Precentor: Officer Rank 7 (in the WoB and pre-schism Com Star, demi-Precentor does not exist as a rank, rather it is an unofficial title for a senior adept with more than 10 years sevice or a junior precentor with less than 5)
Level IV: Precentor: Officer Rank 10
Level V: Precentor: Officer Rank 14
Level VI: Precentor Martial: officer Rank 15 (as the equivalent to a khan or house lord, this should generally) not be attainable during char creation)
Note that these are just my thoudhgts, if you want something more concrete and official ask the writers (if they say use whatever works for you refer back here)