This.Now that you have the carpentry approach, here's the plasterers' approach.
Cut all the crown for perfect angles, perfect fit, glue and nail together as appropriate, put in place and nail along ceiling.
Fill the gap with 5 min setting mud.
Perfect angles, no gaps, no messing around.
If the top back of the crown is snug to the existing ceiling / stair intersection, the area that would usually be a right tringle chase behind the crown will be a tight sliver wedge shape. The bottom of the crown will only be 3/8" to 3/4" off the stair underside.The gap is going to be about 3”.
Ok, look at this. I printed a random crown molding profile full size and drew some reference lines. The image i downloaded and printed had the 2-1/8” and 3” reference marks and i confirmed they were accurate after printing.
I then drew the ceiling line flush with the top of the molding, and an imaginary vertical line at the back of the lower part of the crown. I then drew a 7” rise/11” run line A to indicate the assumed underside of the stair enclosure. Note that we don’t know the size of the installed crown or the slope of the stairs, so these dimensions are reasonable assumptions. Because someone asserted (falsely in my opinion) that the stair slope is 45* I also drew line B.
Since the sketch is full scale, the gap at the bottom of the molding can be directly measured, and it varies between 3-1/16” (what i had stated initially) and 1-7/16”. Both potential dimensions are far too great to be packed with any kind of joint compound or caulk.
The molding profile was taken from mouldingsone.com
View attachment 521729
Good tracking thus down and drawing it out. One detail, I said 5 minute, not joint compound. 5 minute is plaster based. I absolutely can fill that.Both potential dimensions are far too great to be packed with any kind of joint compound or caulk.