: 1. What proportion (%) might one consider for the reduction, and
: 2. are there any specific relationships (i.e. beam vs. length (@ waterline))
: that should/must be adhered to in order to optimize stability/performance?

One option would be to scale her whole boat down. The magic number is 0.87. You could use an even lower number (up to 0.79( because the Guillemot was designed with a displacement of 240 lbs (if I remember correctly). How did I get those numbers? The weight change is linear but the volume change is cubic (L x W x H = V). So to change the boat from your weight to hers divide her weight by yours and raise it to the 1/3 power (take the cube root):
120/180^(1/3) = 0.87
120/240^(1/3) = 0.79
Simply scaling it by 2/3 (120/180 = 0.667) would give a displacement of only
0.6667^(1/3)x240 = 71 Pounds!

Another option would be to use the full-size forms you'll make for yours and use them for hers just change the distance between them (say 11" versus 12"). Nick's book has a section which explains it beautifully.

Not sure about question #2... If you scale the whole boat uniformly, all the design ratios stay the same. I have a limited background in naval architecture (2 classes in college about 15 years ago) and only with play with with boat design on a limited basis. Others on this board would know better. They do it everyday.

Hope it helped. Best of luck.
Dan

BTW: The first kayak I built was a Guillemot. Pretty design.