One problem with using zero wind (if say the headwind is 5kt, and you want to go conservative) is that you can't get 50% of zero.
If, as you line up, you notice the wind is now 2kt tail, in theory you should either recalculate or go TOGA.
Especially where I tend to take off, where there seems to be a tailwind at both ends of the runway, I would probably have put in 4kt tail (which would allow up to 6kt actual). Going conservative by about 10kt seems reasonable to me.