@Yuugi's:
Davy Back Fight through Enies Lobby (#325) fits the formula of the past two 'seasons' FUNimation has followed. Keeping in line with that, I'd prefer to look at…
Season Four (Davy Back Fight, Water Seven, and Enies Lobby):
First: #206/207-219 (14 or 13, depending on what FUNimation does with episode #206)
Second: #220-232 (13)
Third: #233-245 (13)
Fourth: #246-258 (13)
Fifth: #259-271 (13)
Sixth: #272-284 (13)
Seventh: #285-298 (14)
Eighth: #299-312 (14)
Ninth: #313-325 (13)
Season Five (Ice Hunters, Thriller Bark, and Sabaody):
First: #326-339 (13)
Second: #340-352 (13)
Third: #353-366 (13)
Fourth: #367-379 (13
Fifth: #380-393 (14)
Sixth: #394-407 (14)
Season Six (Amazon Lily, Impel Down, Marineford, Post-Marineford):
First: #408-421 (14)
Second: #422-434 (13)
Third: #435-447 (13)
Fourth: #448-460 (12)
Fifth: #461-474 (14)
Sixth: #475-488 (14)
Seventh: #489-503 (14)
Eighth: #504-516 (13) Assuming that #516 is the last episode before the time skip.
Season Seven (Fishman Island)
First: #517-
My opinion, 206 will be online-only/available on Collection 8. That frees up a bunch of episodes and makes this much easier:
Season Four: Davy Back Fight, Associated Filler Arcs, Water 7
First Voyage: 207-221 (14)
Second Voyage: 222-235 (14)
Third Voyage: 236-249 (14)
Fourth Voyage: 250-263 (14)
Season 5: Enies Lobby
First Voyage: 264-277 (13)
Second Voyage: 278-289 (12)
Third Voyage: 290-301 (12)
Fourth Voyage: 302-313 (12)
Fifth Voyage: 314-325 (12)
Season 6: Ice Hunter, Thriller Bark
First Voyage: 326-336 (11)
Second Voyage: 337-348 (12)
Third Voyage: 349-360 (12)
Fourth Voyage: 361-372 (12)
Fifth Voyage: 373-384 (12)
Season 7: Saboady, Amazon Lily, Impel Down
First Voyage: 385-397 (13)
Second Voyage: 398-410 (13)
Third Voyage: 411-423 (13)
Fourth Voyage: 424-436 (13)
Fifth Voyage: 437-451 (14)
Season 8: Paramount War
First Voyage: 452-463 (12)
Second Voyage: 464-476 (13)
Third Voyage: 477-489 (13)
Fourth Voyage: 490-502 (13)
Fifth Voyage: 503-516 (13)
This also gives an even number of Voyages to be condensed into Collections, ending with Collection 30. It also ensures that if FUNimation wants to stop at any point, it would be at the end of an arc as opposed to cutting just short like what happened with Season 3.