A wall that isn't square on the deck will never be plumb in the air. Learn the diagonal squaring checklist before sheathing, string line plate leveling, and 2x4 push-pull turnbuckle bracing.
Both diagonal corner-to-corner measurements must be 100% equal before nailing exterior OSB/Plywood sheathing.
Place 3/4" gauge blocks at both top plate ends and pull a string line. The middle must read exactly 3/4" across the entire wall.
Install diagonal 2x4 braces at a 45° angle to the deck with 2x4 kickers every 10-12 feet to micro-adjust plumb lines under load.
Tack the bottom plate straight to your snapped chalk line on the subfloor with 3" toenails. This anchors your reference baseline firmly.
Hook your tape from Corner A to D, then Corner B to C. If diagonal A-D is longer by 1/2", kick the top plate 1/4" towards the short corner until both match to the 1/16th.
Fasten the 4x8 plywood/OSB sheathing sheets while the wall is flat and squared. The sheathing permanently locks the wall in a perfectly rigid 90-degree rectangle.
Enter your wall height and wall length to calculate the exact corner-to-corner diagonal distance for 100% square framing.