on 66% rate server got on 40 tries from +4 to +5:
FSSSFFFSSFSSSSFSFSSFFSSSSSFSFSSSSSSSFFSS
on your example when you mean FFS for +5, means i need to wait 2 Fails and then real enchant ? and on +6 3 FAILS and then real enchant ?
from what i test, really dont see a pattern , but the rate is pretty close.
Lucky