Ok, after botching the install of one of my replacement decals on my new tail section I got another one and this time I done it right. Since the right (brake) side was on right I measured by it to align the other side. In doing so I came up with some measuring points that make replacing the ZX-10R decals as painless as applying decals can be. Thought I'd share it with everyone in case someone needs to do this some day.
To align the leading edge (closest to front of bike) of the decal on both sides so both of them are the same distance from the front and back of the tail, use some masking tape (or any other easy release tape) and go straight across the tail section lining the tape up just ahead of the point of two little grey seat cowl dampers that are on top of the two brackets located below the seat/cowl. I used 1/4" blue 3M fine line masking tape and had the rear edge of the tape approximately 1/16" forward of the tip of the dampers on both sides. On the sides of the tail just wrap the tape down as straight as possible, keeping it inline with the piece across the top as seen from the side of the bike.
I then used a cloth measuring tape and measured 1/2" from as close to the center of the top body line on the tail as I could get with my eyeballs judging where the center was. I laid another 8" or so piece of the same fine line masking tape on top of the first piece and aligned the bottom edge of the second piece at 1/2" from the bodyline center and then pulled the tape fairly tight and laid it down about 7" back with the bottom edge at 1/2" from the bodyline center.
With the front edge alignment marked with the tape across the tail and the top edge alignment marked with the piece 1/2" below the body line it was as simple as peeling the sheeting from the sticky side of the decal (leaving the clear backing on the front side) and carefully laying it down with the decal's edges aligned with the blue tape. Here's a pic with the replacement *replacement* decal installed properly and the blue tape still on there to illustrate the above instructions.
Note: In these pics the reflections make it sort of appear that the decal is not aligned to the body line above it, but rest assured that it measured 1/2" on both ends of the decal.
And the decal as close to perfectly located as I can humanly get it...
...and with a fresh coat of Zaino to restore the luster after stripping the Zaino before the install so the decal would bond.