borderless printing on an Epson 9890 – solved

I’ve had trouble getting a 24” borderless print on my Epson 9890. No matter what I did, it was off by at least an eighth of an inch, (and often more) cause me to lose details on the edge, not to mention ending up with a print that was less than 24” wide.
 In many cases, this would not be much of an issue, but my work is art created at a specific size, and losing the edges was not a solution. I hunted around the web (only to find others with similar troubles.) Naturally, I followed the instructions in the book.  In fact I spent most of the day on it, talked with Epson Pro support, and then I finally recalled for my distant past experiences,  that the Epson Driver is “non-trivial” and mucks about with numbers internally in ways deep and mysterious. 
I’ve been printing on Epson’s since the original Stylus 2000 photo printer days, and have seen this before. For example, in once case, if you specified an image size longer than the driver could handle, instead of a cropped image, you’d get a  tiny little image rotated 90 degrees and  bleeding off the left edge.
Today, if I put an offset of 0.150 into the printer dialog, and come back and look at it, it will be 0.138. Or look in the book and explain their recommendations for borderless printing…(which, BTW, don’t work.)
There is something arcane going on and its been that way since day one. I’d forgotten about it because I switched to ImagePrint RIP a couple of years ago for my 3880. However a couple of weeks ago, I spent all my cookie jar money on a 9890, and can’t afford the $2.5K for the new, larger, license of IPR.
Enough backstory.
Here is the solution in case someone else is looking to do borderless 24 on a 9890:
24″ roll printing  borderless.
Create a custom paper size of 24.23 (x 36.23)
make the image   24.03125 (24 and 1/32″)
in the print dialog, set the top offset to zero, and the left to 0.1625 
in the printer setup, choose Borderless (retain size.)*
THAT finally got me to a level of borderless precision that was within the range of paper-slop.
Hope that helps someone.
* Do NOT mess with these numbers. They have to be exact because of Epson’s internal twiddling with them. (For example, when you put 0.1625 in the left offset, it will change to 0.152 if you come to back to look at it.Don’t try putting in 0.152, as it will change to 0.138…)