I’ve used an amazon box to try and get a right angle between your head and wall. I think it is hard for one person to do accurately. And with you - it might be hard with two people without a stool :). I try and stand up straight and look straight ahead and trap the box against the wall. Sometimes it will move, but usually I can get out without feeling/hearing the box move. Then just use a pencil and measure to the floor.
Doctors use those “stadiometers” - which aren’t cheap.
sometimes when you put a box on your head it can slant at an angle without you knowing since you cant see it. Thats why some times I measure myself at 6’5.5 up to nearly 6’7. So it can be hard.
For measuring height by yourself use a chefs or similar large knife laid flat on the top of your head pointed toward the wall. Make a small stick in the wall and use that as your reference mark.
Re growing after 30 height can vary by over an inch a day from morning to night, especially for tall people. Losing weight and/or working out will also make you look noticeably taller due to a more slender profile, and improved muscular posture.
I was 5’8" and a fraction for most of my adult life. I moved to Wyoming at age 42 (I’m 48 now) and am now 5’9"+ every time at the doc’s office. Higher altitude, and less barometric pressure compressing me. Either that, or the lack of stress from the slum cities made reinvigorated me.