I do it the same way with a couple refinements. I squeeze the neck of a fired case until it is slightly oval to firmly hold the bullet. I then use a magic marker to mark the bearing surface of the bullet and partially insert the bullet and then carefully chamber the round and extract.

Sometimes the bullet will stick to the rifling and partially pull back out of the case giving a bad measure. If this happens the cartridges neck end will scrape away the marker and reveal the actual max depth the bullet is pushed back.

It is best to trim the brass without chamfering the inside of the case neck when doing this to ensure that the marker is effectively scaped away.

You can then measure the cartridge to verify its OAL either to the tip of the bullet or the bearing surface depending on which you choose to use as you standard of measure.