How cold can I go in a 40 degree sleeping bag?

Probably to about 40. Sleeping bag ratings are usually "optimistic".

What sort of bag (brand, construction, fill, etc)? What sort of weather? What's underneath you?

I saw your Uinta videos on Youtube. My family and I camp in the Uintas a lot. I plan to do a lot of backpacking there this summer. I've done quite a bit of day hiking/bushwhacking but haven't really gone overnight since boy scouts.

Mat,

I've been looking into the UL Super Spiral Hugger #5, Summerlite, and Megalite.

I haven't decided between closed cell vs. neo air yet. Probably NeoAir though.

That is awesome! It is beautiful up there.

I have a Marmot Atom 40* bag, and a NeoAir size reg. I sleep pretty warm also, and have had my Atom down to 28*f. I was wearing a mid merrino wool base layer, cotton undies, never syn. in the bag, I found that there is not as much humidity inside the bag when I wear the cotton, or natural fibers vs syn. Goose feet down booties,fleece balaclava, we were in a 2 person tent with no rain fly, all netting, and a slight breeze. In the middle of the night I put on my convertable hiking pants, and my light jacket, don't remember what jacket I had, but I know it wasn't very heavy because we were not expecting the temps to get that low. I was a little cold for the last few hours of the early morning, but I know if I had better layering I would have been warmer. I have had this bag down to 32* quite a few times with no problem, with a better layering system. Now I use the W.M Alder 25* bag, haven't had it below 32* yet, but man this thing will keep you toasty warm.

Correction: I was looking at Super Spiral #3, Summerlite and megalite.

I was wondering if sleeping with clothes that I plan to pack anyway would help me go with a lighter bag.

Sounds like I better stick with a bag with at least a 30 degree rating as I expect to regularly hit low 30s and some high 20s at night with this setup.

whatever the en-rating is on that bag …. thats a rough baseline
'

That is kind'a like asking how far is up.
It totally depends on what types of internal layering you are able to fit into the bag without overly compressing the loft. It depends on any external bivy. It depends on if you will be sheltered in a tent. It depends on what type of pad you will be using under you. It depends on if you have a partner with you.

Example: I have had my 40F degree bag at 40F and been comfortably cool. I have also had it at 20F, ziped together with my mate, and been too warm. It depends.

The Super Spiral #3 is a 30 degree bag. Im a warm sleeper and have comfortably had mine to 28 degrees wearing just a light wool base layer top and boxer briefs. I use a Big Agnes Insulated Aircore pad.

I'll be solo, in a Tyvek Sublite and most likely on a small NeoAir with no Bivy.

I've slept down to about 10* in my 35* Western Mountaineering Highlight bag at 12k. I had all my down gear on though including my hat and booties and my MSR e-bivy and a 3/4 prolite pad. I stayed warm enough for three nights all ranging from 10 to 25 at night. So take that for what you will. I think it depends on the person, but I believe you can give any bag a better rating with the right layers with it. If you don't have a bivy sack or won't be wearing anything but base layers then I wouldn't give a bag much more than 5 maybe 10 degrees than what its rated at.

Probably around 35F give or take. That is pretty standard gear, 'cept for the short pad. Provided you can put up with the condensation and a closed door.