Anyone with any credentials have an opinion? (no offence mate)
Actually those calculators aren't bad at all. Keeping in mind that they assume things and there aren't many variables.
I'm 170 lb, 5'7" and ride a 159 cm Freeride board. Tried an alternate calc and it's right on. I used to ride a 153 Freestyle board and the calc and it said 153 as well. I know it's just a coincidence but it seems pretty accurate.
Of course you have to take that number with a grain of salt, adjust for blunted tips, and adjust for specific needs.
Ulimately here a few centimeters is nothing. I would drink screech from a boot if somebody could tell the difference between identical boards that were 3-4 cm difference in length, by feel. I highly doubt I could tell the difference.
I felt a moderate difference going from a 153 to a 159, but my stance width also opened up by about an inch, and I went from a medium flex board/boot/binding combo to a stiff setup.