First off, I'm terrible at math and embarrassed to ask. On a Utube video some guy was preparing an area to build a shed floor. He used stakes and twine to outline the space. To make sure it was square he started at an outside corner, measured and made a mark 3' down one side and 4' down the other, then across the inside from mark to mark and said it should be 5'. That tells you it's square, or not and to adjust and measure the other three corners the same way.

There was no info on what size area he was squaring up. I want to prepare a 6'X10' area. Do I use the same 3', 4' = 5' forumula too?