No matter day hike, overnighter or multi-nighter, lighter is better. After 1st starting out and bringing all kinds of lb's I did not need or use, I started realizing what I could cut back. I think that it helps to go out and try differing configurations to seewhat works best for you and the environment you will be in. Fortunately around here in N GA, there is usually water and I carry my Katadyn Hiker so I can just pump it straight into my Platypus.
Of course my configuration will vary depending on weather and duration. I also try to multi-purpose as many things as possible.
This has worked nicely in the past.
Set up 2 blankets on the floor and put ALL things you are considering on taking on the 1st blanket.
Put
ALL the things that you
KNOW you would be a fool to go into the woods without into a seperate pile on the 2nd blanket.
With the remaining pile, decide which items you
STRONGLY believe you
NEED and put those in the pile on the 2nd blanket.
With the remainder, decide what you
VERY STRONGLY WANT to bring and put that on the 2nd blanket. Go over these items carefully, this is where most of the wasted weight will be.
Take the remainder and weigh to see how much you have dropped and put it in the garage.
I then usually go back to the pile I plan on taking and see what I can multi-purpose and remove anything that I fell Iffy about. When backpacking, I'd much rather want it and not have it than have it and not want it. As I already stated, I already have everything I'd be a fool to go into the woods without and can survive indefinately with just that.
But there is no magic number. It is all about trial and error and error.
One thing I do believe in, even when trying to light, is a good pack with good support on the hips. Most UL packs do not even have a waist belt, which reduces the weight of course, but it puts all that weight and strain on your shoulders. A good pack with a hip belt will put 80% of the weight on your hips where it belongs. Take a decent hipped backpack and an UL w/out, fill both with 20 lbs and see which one feels better at the end of the day.