Pathagareon theroem a squared plus b squared equals c squared. 3 4 5 … 6 8 10
If it’s important to have a reasonably square room (tile, stone, cabinetry, etc.) I’ll fir out the walls. I’ll do the same thing if they need to be plumb. Nothing worse than seeing a tiled floor running off angle into a wall. Of course there are “cheats” but sometimes it’s more efficient to correct the walls instead of “funny” cuts.
First pick your longest wall… that’s your control “line”… measure 8ft out from the corner and make a mark on your control line… go back to the corner and measure 6ft up the other wall, make a mark.
The distance between the two marks must equal 10ft exactly… if the diagonal is more than 10ft, bring your 2nd mark “in” slightly until the diagonal measurement is exactly 10ft…
if the initial diagonal measurement is less than 10ft, that means those 2 walls are not square and the angle is less than 90°…
To fix this, you’ll need to redo the entire process, but, start your control line 1/4" away from the corner, and make sure you measure your 2nd line from that point which was moved 1/4"…
Keep increasing the control line starting point by 1/4" until you get a square measurement to your control wall.