What he does in the video is get the exact diameter of the bore.
with slugs, you want just enough surface contact with the lands for a smooth fit.
not too tight..not too loose ..all for better accuracy and consistency
with pellets, there is a forgiveness on using a too tight/loose size due to the shape of the projectile .. slugs are less forgiving
so when he is taking the measurements of the grooves on the slug that has been pushed through the bore, that is the diameter size of the slugs that best suits that bore. weights are another thing that you will have to test out. All bores are slightly different, like a fingerprint, so where someone will tell you to use a certain diameter in the rifle you are using.. yours might vary slightly, so use their suggestions as a marker of where to start your tests.
A conpany like Nielson has a variety of weights and diameters to aid you in finding the right one
Nielsen Specialty Ammo manufactures high quality hunting and precision target air gun bullets for PCP air rifles and pistols. Swaged and cast slugs.
nielsenspecialtyammo.com