DrPhil wrote "Since I am not using readyloads, how could I possibly keep 400 sheets of FP4+ separate from each other? Especially if each sheet has its own individual developing time. Three boxes for N, N-1, and N+1 seems easier in this regard. Does anyone have a suggestion here?"
I've described my method to several people, but most don't like it, perhaps it will work for you. I use small Post-it notes on all of my holders. As I make an exposure, I write all pertinent info on the note. When I unload the holders I simply stack the Post-it notes one on top of the other in the same order the sheets will be loaded into the box and those notes stay with the box until I'm ready to develop.
When I'm ready to develop I pull all the Post-its for a specific manner of development noting their order in the stack: for example, 1st, 3rd, 7th & 15th. I then count through the stack of film pulling these sheets.
I know this sounds complicated, but it really isn't. It just requires a little concentration and attention while doing it. I've developed thousnads of sheets this way and never had a failure, well not a failure due to development error.
The one failure I had was while shooting in the Grand Canyon during quickly changing light. I was very rushed and inadvertently forgot to remove the Post-it before pulling the dark slide. As it went through the light trap it tore and I though I removed all the pieces, but one eluded me and became lodged in a fold of the bellows. About 40 sheets later I noticed a bug on the inside of my groung glass and removed the back, when I did I saw the small piece of paper! The result was about 40 sheets of film with flare or some sort or another from that tiny piece of paper. Needless to say a lesson learned, but I haven't abandodned Post-it notes. Good luck!
Regards, Pete