IF you do have a scale do you pack items you already own based on their weight? This spoon weighs less than that spoon)
Absolutely- every little bit counts!
do you find the scales prompt outbursts of buying lighter stuff?
Like Jim- sometimes. For a crazy trip where I'm getting really low packweight, the answer is definitely yes. And when paring down items in general, yes- it helps to have the real information.
Do you set targets as far as pack weight and yank stuff out to meet it?
Sometimes. For example, I've got a set base pack goal of 4 pounds for a summer insano trip I'm taking. For that one, I carefully select items, trade things, etc. But for general trips, most of my scale work is done- my kit is pretty set and I know how light it is. I simply put the items together and hit the trail.
I find target pack weights to be great motivation to find alternatives.
Absolutely- me too. And I really enjoy the challenge, actually. It's an extension of my backpacking- almost a second hobby at times. First I was at 10 pounds, then 8, then 7, then 5.5, and now 4. Or should I say, I'm at 28 with my son on my back!!!
Trust vendor weights? It's in my nature to measure things myself.
The vendor weights are often way off. Even Bozeman Mountain Works is off a little sometimes, and they're as accurate as it gets. My gear lists have my weights on them- not claims by manufacturers. When my pack weighs 4 pounds, it's FOUR baby!
A scale is cheap. I got mine on eBAY. Get one and join the Ultralight Fanatics Club!