Coming from 8th (where we pretty much all use Chris' method) I notice that 10th guys have a wide variety of ways of measuring it, mostly by placing the car on a ride height block and then using a droop gauge.
If you're copying someone else's setup, best to use whichever method they used. If you just want a record for yourself do whichever you find easiest. Myself, eye to eye on shock with a set of vernier calipers is easiest and most reliable. And yes, it works just fine if you have droop screws to - less droop = shorter shock.
A quick lesson from Adam Drake (who's quite handy!)
http://www.youtube.com/watch?v=H99PKqsUZjA