Ah yes, the good old 2x4. My own technique requires about a five foot length. Clamping the frame in the stand, I hold the board against my chest and pull on the head tube. I pull until I feel a little "give" and measure. Repeat as needed.
I've done the floor method as well. I never felt it was subtle enough. Unless the frame was grossly bent. The I block it up and jump on it!