Gotta add to what VG says about mixing. That's okay and fine for plinking, but consistency has its merits.
My thinking on lengths is to have the "internal" of the neck all the same. Fire the brass once, that gets them all fitted to the chamber. You can still have pretty good accuracy with your "fireform" loads.
Then when you trim, you trim all the same. You can either trim before necking, or trim after necking. Just make sure they are all in the same sized or unsized condition.
Some cases will miss, or just skim a little, some will really hog off some metal. Then the chamfering (I use either a rotary file or grinder burr) is done in the drill press at the same depth setting.
This all gives an identical internal length, the same amount of neck grab area for a batch of bullets all loaded to the same OAL.
Makes the bullet release all the same.
The only cases I'll cull are the real "shorties" that don't take a hit from the chamfering on the inside of the neck. If they skin just a hair, that's enough for working loads.
