On page 16 of the manual
Pentax Imaging - Manuals & Literature
- there's a chart of Guide numbers at the various power settings and zoom head settings. Tape that chart to the flash head. With a little bit of practice you'll learn how to make the adjustments you need pretty quickly. Keeping the flash head set at one focal length would probably help a lot as well, since that's one less variable to muck up the works.