feeling lazy, so in brief:

spin the cone down by hand, checking the fork as you do so to feel the point play is gone.

hand tighten the locknut down on top of it

place heatset wrench onto cone, 2nd wrench onto locknut
it's convenient (but not required) if you place them such that both handles can be grasped simultaneously with a single hand

with the fork held from rotating, torque the two wrenches into each other, being careful that each one moves the same number of degrees against each other -by doing this equally, the cone maintains its preload position, since the motion from locknut vs cone threads cancel each other out
feel for fork play as you do this, and bias the wrenches if needed (noted earlier: one hand squeezing both wrenches together. frees up 2nd hand for testing play)

so, all the hard wrenching force is done in one motion at the very end, greatly reducing the amount of back and fourth marring of the flats
the preload was mostly found by hand in the first steps

overall its a very quick and smooth process - no back and fourth

yeah, it takes practice. work in an LBS for a while, when you need to get things done both quickly and correctly you figure these things out.

i guess, outside of that context there's nothing 'wrong' with doing it the fumbling slow way - other than chewing up the flats.... you still get the headset adjusted in the end