I can't. I'm half german and quarter bohemian, my father is an engineer, my grandfather was an engineer, my great grandfather was an engineer - it just isn't in my nature to not be exact and precise. I did a group/club brew with a spontaneous brewer once and it drove me insane.
I'm Swiss, German, and Danish, I plan and usually try to stick to it, unless there was an inventory mistake and I have to change hops or something or if I'm an ounce or two off of base malts, but I don't worry about much about it. I drive bluesman crazy when we brew together, because he really does get down to the details and I don't care as much.
I used to be that way, but I got older, and less concerned.