For most "normal looking steelhead", a buddy and I figured out that once they're over 25 inches and less than 34" - it's a pretty close guess that you can drop the first digit of the length (from 25-29) and replace a 3 with a 1 for the 30+'ers and it's close to the pound weight. i.e. your 29 inch fish probably weighs close to 9 pounds.
Once they get over 34" they tend to broaden. I've seen quite a few fish that weighed in the high teens that were only 34 - 36 inches long.
-Matt
www.slammertipup.com