For a 6-foot tall man whose urethra lies at his mid-point (3 feet off the ground), standing in front of a toilet that is exactly 1.5 feet off the ground with an opening 1 foot wide, there exists a range of 18° to both the right and left of true vertical in which he can safely aim his stream.
And as we all know, 18° is 20% of 90°. So, the hypothetical male has a 20% margin of error to both the left and right of true vertical. This probably sounds like a reasonable margin of error, BUT until now we have completely discounted the organic shape and surface of the urethra and the head of the willy.
And the urethra's terminus at the tip of the wong is of vital importance! At this junction, the skin parts at approximately 45° to the right and left. This angle in combination with the adhesive properties of water (urine is approximately 95% water) results in a substantial urinary course correction as the water/urine pressure begins to drop toward the end of a bout of urination. In other words, as the bladder empties itself, pressure drops, and urine tends to "stick" to the sides of the urethra/johnson more actively (i.e., as the "straightening" effect of water pressure diminishes, the effect of surface adhesion becomes more pronounced, thus encouraging urine to follow the natural curve at the head of the schlong).
So we now have the potential for a 50% (45° being half of 90°) course correction working against a 20% acceptable margin of error. And this has the makings of disaster...or at least splatter.*
But really, we should have the decency to at least wipe off the seat... ;-)