Getting precise angles

I use a centering ruler for quick measuring. I use a steel ruler for cutting and scoring. there is a difference between cutting and scoring.

Scoring cuts the top layer but does not cut the bottom layer. You can score all paper and cardboard but you need to know how your knife and blade cuts. Sharper blades only need a tiny bit of weight. Dull blades are not recommended for anything. In fact, dull blades are dangerous for many reasons; it's hard to cut and judge the pressure needed which results in slipping, cut fingers and many other accidents.

Scoring lines need to be covered up later since they leave obvious gaps in the surface.