Is there a max ratio that you need to stay under when you notch a newel post in order to secure it to the joists?

[Ivan Turbinca]

I am talking about this situation

Here is my situation:
I have two posts that I am dry fitting right now and one of them will be sandwiched between a structural joist and the stringers for the stairs that go to the basement. This will be screwed to the joist and to the stringer. from the side of the joist opposite to the viewer. I have access there, under that there is a crawl space.
The other one (on the left int he picture) will rest on the joist on a 1" shoulder and it will probably be screwed to it from the side. I have like 1" of drywall there which I can cut through and then notch the post to be flush with the wall OR if 1" thickness is not enough it will have to protrude out of the wall like in the above picture

Here is a closer look at the sandwich, there will be no void spaces there there is a 1" trim piece of white oak that will support the glass panels and the posts will rest on it

 

[C.J.]

Usually it’s a 50/50 split. Could you get away with something less, prolly. Would I cheat it if that was my house, prolly.

 

[Ivan Turbinca]

Any recommendations for the screws I should be using here ?

 

[C.J.]

Lag screws or lag bolts

 

[Ivan Turbinca]

Great thank you

 

[Ivan Turbinca]

Is it an option to use an auger bit, drill both the floor and the newel post and then glue metal pipe in that hole and then the post to the pipe with ...epoxy? Someone inspired me by asking if I could use an 1" dowel there but I already had that and it id not feel great. It always felt shaky

 

[Ivan Turbinca]

OK here is the cut for the above last picture.
There are some challenges regarding making this larger, please see the numbered diagram below
Right now the tenon is 2"x1.5"x6". Is this going to hold ?
I am going to use PL Premium and maybe structural screws from the left side, see the last picture, only that side is accessible

What is not seen in the picture is some large nails ❷ that are used to connect the two structural joist that support the floor at the top of the lower stairs.
❶ are also large nails that attach the stringer to ❹
The 1.5" side (the narrow side) is what is worrying me and the problem is that I can not expand (cut deeper) toward the stringers as I am afraid that the stringers ❸ will be weaker and the stairs could collapse
It I try to enlarge it the other way I am going to cut into a structural joist ❹ that has support from below so that is fine but I am going to get into the nails❷. Besides that the nails hold ❺ connected to ❹. If I expand the cut toward ❹ the nails ❷ will have less material (pressure applied to them) and they will disconnect as the stairs is leaning on ❺ via the stringers ❸.