Ideally confirm that the marks are level with a laser level if you have one and revise if.
How to build a attic knee wall.
Use your saw to cut along this line.
Step 1 measure the attic to build your walls to the correct length measure the angle of the rafter by laying a piece of 2x4 inch lumber against the side of the rafter in a vertical position.
Install insulation without misalignments compressions gaps or voids in all knee wall cavities.
Push the knee wall firmly into place.
A knee wall is a short vertical wall roughly two or three feet high that blocks in that useless triangular space.
Install a top and bottom plate or blocking at the top and bottom of all knee wall cavities.
But you do so at the expense of floor space.
The higher the knee wall the greater the amount of useful wall space you create in your finished attic.
Install an air barrier on the exterior of attic knee wall insulation and to block open floor joist cavities under attic knee walls.
Measure and mark the rafters at each end of the attic with the desired height of the knee wall.
Face the knee wall with the appropriate size of drywall.