It's fairly simple... mostly just to lock me into a rotation so I don't "accidently" fudge it in my favor
When ever I end a patrol, I go to the crew management screen by clicking on the crew picture on the desk in the office. I take the last digit in the current renown value (upper left of the crew management screen) and start counting from the upper left crew position and progress across the top row until I hit that number. That's the first guy I whack.
Then in 1942 I plan on losing ~25% of the crew... or 1 out of every 4. So from whack #1, I count 6 <whack> then 3 <whack> then 5 <whack> then 4 <whack>, and keep going with that series - left to right, top to bottom - until I get to the last guy, usually the AA gunner.
If it's '43 or later I simply adjust the formula for a 1/3rd crew rotation.
Then, after all the whackings are finished, I apply promotions to the remaining crew to see where I am. I suppose I should apply promotions first - before I determine who's getting whacked - but there it is... a little leeway I allow myself.
Next, I back fill from the base repple-depple using only ZERO cost replacements. Sometimes, a guy I just whacked shows up in the pool at ZERO cost (they go into the pool as soon as you let them go, but you can tell because their stats show 1 or more patrols completed). If that happens I usually pass them by. That means all my back fills are either grade 1 or grade 3 seamen, or grade 10 officers (ensigns).
The only time I violate that ZERO cost replacement policy is if - after the whackings - I'm short of CPOs. I typically keep a minimum of 1 CPO in the engine room and 1 in each torpedo room. So if I need one I'll spend the renown to get one.
Doing this with the crew keeps things interesting and saves renown points for sub upgrades.
JD