Designing necklaces using a flat board doesn't seem to represent what the necklace looks like when draped. Is there a tool the professionals use to create necklaces that are the right length? Or are there tips you can give to make it work with the design board?
Professional jewelry designers use beading design boards to plan their necklace designs. These handy tools help determine where beads should be placed before they are strung on wire or cord. Follow the numbers imprinted along the design board's edges for size--they're inch marks, so they should help you gauge the proper length. Also consider that the clasp and possible extender chain will add at least an extra inch, or so, to the final length, so be sure to include this in planning the design.
If you'd like to get a sense of how your design looks draped, string your beads on the beading wire without securing the ends, attach a Bead Stopper™ to each end to temporarily secure the beads in place, then hang the strand on a necklace pedestal display. This way you can get a sense of where the focal and other beads will lay when worn. Once you are happy with the drape of your design, secure the ends as desired.