How much space do you like to have between the front of a loaded cartridge and the magazine end? Does it matter as much for bigger bore rifles? I generally don't load so the bullet nose touches the mag end, but is there some rule of thumb for something like a 458 Win Mag/458 Lott/458 AR? Do you want to leave maybe .050" of free space, or is this too much and would result in too much battering of tips during recoil?

No rule of thumb. Seat the bullets so the cartridge pushes down into the magazine smoothly and then feeds reliable.