Everyone is different.

I use this method to inspect drawings on site or during presentations.

I know my fist width is 100mm, sejengkal is 170mm, and my belt height is exactly 1000mm.

I also know how to use them for scale. For example, human height (1.70cm) is exactly my thumb's width at 1:100 scale.

But like I said, everyone's different. What I use for measurement would be different from yours.