don't worry too much about numbers people give for baseweights, I can't really get mine under 12 unless I'm going overnight or it's really hot, just work on the big items. Most non ultralighters would consider my pack weight ultralight, and they are right. The notch is a good start if you use trekking poles, not sure I could get much enthusiasm for carbon fiber trekking poles, that would be about the last place I'd look to save weight, cracking poles isn't my idea of fun.
Check out packs if you have a backpacking store near by, like REI, and just look at them to get an idea of the volume.
I found about 35 liters is decent for a nice light setup, pack that is. 50 liters is big enough for anything, particularly a bear cannister, if you need one. If I used a bear cannister my base weight would be about 14 pounds, give or take.
Once you are between 15 and 12 pounds base weight you can sort of see what you like, and what you think is worth it. Having spent a lot of time in a tent in the rain, I find the notion of a bivy in that circumstance comical, or painful, for example, I'd never do that.
So just take what people here say with grain of salt, ultralight has a lot of variations, and you can figure out which work for you. In summer a myog synthetic quilt would be a fun project, to use in summer I mean, but it all comes down to your time vs the little bit of money you might have saved making it. Depends on you, what you like to do. And your sewing/learning ability. Price something like enlightenment quilts, they are made by a guy here, and his prices are more than fair. If you like projects and enjoy the process of making things, make it, but as I've learned, really, I have spent far more experimenting on materials and designs than I saved, but it's a lot more fun making the stuff so I've come out ahead.
One neat ultralight trick is to store your sleeping bag in a turkey baster bag, then cram it all on the bottom of the pack, no stuff sack that is. Not that the turkey baster bag saves you much weight over a light stuff sack, it doesn't, but it does fill in the bottom of your pack really nicely, and it keeps the sleeping bag from getting over compressed.
Depends on how many days and what conditions you are wanting to go for too.