I ended up using a Craftsman multimeter with a duty cycle function. On this meter it reads the duty cycle on a different segment of the square wave on the test port so the numbers were 100 minus what's in the spec book but I think I've got it tuned in pretty good.
Here's a good thread:
http://forums.pelicanparts.com/porsche-911-technical-forum/874460-help-lambda-freq-valve-test-please.html