Assuming it'd be too hard to change the level progression, here's a way to tweak the rank progression to be somewhat more reasonable:
- 1: Cadet
 - 3: Ensign
 - 10: Lieutenant J.G.
 - 20: Lieutenant
 - 30: Lieutenant Commander
 - 40: Commander
 - 45: Captain
 - 50: Commodore / Rear Admiral, Lower Half
 - 55: Rear Admiral / Rear Admiral, Upper Half
 - 60: Vice Admiral
 - 65: Admiral
 - 70: Fleet Admiral
 
If the level progression can change too, here's what I'd do:
- 1: Cadet
 - 10: Ensign
 - 20: Lieutenant J.G.
 - 30: Lieutenant
 - 40: Lieutenant Commander
 - 50: Commander
 - 60: Captain
 - 70: Commodore / Rear Admiral, Lower Half
 - 80: Rear Admiral / Rear Admiral, Upper Half
 - 90: Vice Admiral
 - 100: Admiral
 - 110: Fleet Admiral