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.
Install an air barrier on the exterior of attic knee wall insulation and to block open floor joist cavities under attic knee walls.
A knee wall is a short vertical wall roughly two or three feet high that blocks in that useless triangular space.
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.
Nail down the sole plate the bottom of the wall in three or four places into the attic floor.
Do the same thing for the angled top of your knee wall nailing it into the rafters in three or four places.
The higher the knee wall the greater the amount of useful wall space you create in your finished attic.
Mark the line that the angle forms along your 2x4.
Use your saw to cut along this line.
Install insulation without misalignments compressions gaps or voids in all knee wall cavities.
Measure and mark the rafters at each end of the attic with the desired height of the knee wall.
But you do so at the expense of floor space.