I'm in the midst of constructing my own small HHO generator as well. I'm using the basic design of what is called the Smack booster, but with quite a bit of modification from the original design to correct it's obvious flaws.
For a guideline, I'm using the info/how-to books from a company called Eagle Research,
www.eagle-research.com , to construct mine. Their version of a HHO generator is called the Hyzor. About 3 years ago, I tried a few of their fuel saver devices on an old '87 Subaru gasser I had, and they worked quite well - bumped up the old beast from 23 to over 35 mpg. From what I've seen, their info is solid with no BS, and I've no doubt their HHO design would work as well.
I've no doubt you could easily exceed 30 mpg. With the mods listed in my signature, plus the ORM/SEGR to turn off the EGR which I don't like to publicly advertise, I've been hitting 25 combined/33 interstate with mine. With the proper amount of HHO, and perhaps some of the other mods discussed on LOST, I'd say that it's not outside the realm of possibility to push 40 mpg interstate with a CRD.
ER advises to start with the lowest amount of HHO you can dial your generator down to, and then gradually bump it up until you no longer see a fuel mileage increase - they also say that too much HHO will actually decrease fuel mileage.