Hi,
look at bottom QRH 4.02 & 4.03. it is written to use in flight.
the case in then very simple, if you plan a Flaps 3 approach & landing due to go around limitation, use 4.02, no failure, no factor. it is written as well there is no margin on those distances. the FAA recommends a 15 % margin on that. all that for the legal side.
now you've got an emergency and the runway is just enough long to contain the distance calculated above, it might finish with a global emergency for gear fire (full braking) or overrun
Sound side now, you take 4.04 autobrake low and find the adequate runway