this is a method i have never tried, and i won't buy the book. but it looks easy and it isn't hard to realize that the width of the strip has to be equal to the unfinished leg of the unfinished triangle.
i would add i smidgeon to the width and them trim, but that's me.
if anyone tries this, please let us know how it works out.
http://www.pacinfo.com/%7Ehallock/triangles.html
i have never seen this before.