For me, I require something if for any reason it is needed to maintain what I think is an acceptable level of safety. Every crew will have a different opinion of what is an acceptable level of safety. You can't put figures on it as there are so many variables, are you training? Have you been awake for 19 hours? Is it the first time in in 10 years into that port? Is your anti-skid inop?
Do you have all reversers a working?
It just comes down to personal opinion as does most everything.