The guide number of this flash is pretty low. I haven't done careful tests, but the results I'm getting are consistent with a guide number in the neighborhood of 25 at EI 100 or 50 at EI 400. The second shell is for reference after you start cutting.
Additional shells are for experiments and mistakes.
Please heed all the warnings on this page. For more safety tips, typical schematics, and detailed information about electronic flashes and strobes, see the sci.electronics strobe repair FAQ . I used a nibbling tool to cut a two-cell holder down to one cell, and relocated the positive lead accordingly. Also I think they deliver a bit more juice. This thing is still pretty slow to charge after the first few firings, though.