OK, what did we miss? Since you already load metallics, you have a powder scale. The powder bushing charts only get you close. Besides, different lots of powder can be of different densities. I had to go down two bushing sizes when I ran out of 1989 Hercules Green Dot and opened a can of 2002 Alliant Green Dot. More about that load later.
You say you have the Lyman book. Is it the latest 4th edition? Data does change. It's good, better than DBI's Reloading for Shotgunners, which is OK. You can get more reloading recipes from the powder companies websites.
A basic principle of shotgun reloading is that the shell has to be full. The combination of powder, wad and shot must fill the shell to the point that you can make a proper crimp, neither dished or high. So you don't have much room to vary the load. You'll need a different wad and maybe powder when you change from a 7/8 load to a 1 oz. load. Alliant's data shows Herco for both, might be a good place to start.
You can make a little more space for shot by adding wad pressure, but I'd rather have a combination that just moves the indicator on the MEC, which is about 25-30 pounds. Too much got me in trouble once.
There are inside tapered shells like Remington and Winchester target shells, and straight wall shells like Federal hunting shells. The wads are designed for a particular type, and aren't completely interchangeable. In my case, a Federal 12S3 wad works in a Remington RTL shell, but I must use a reduced charge of Green Dot, compared to the charge of faster Red Dot I use with the Remington RXP12 wad. The Federal is just too tight in the Remington shell.
You can partially cure the bridging problem with larger shot by getting the burr out of the inside bottom of the rammer tube. Then lift half an inch before you drop shot, as MEC recommends.
Download the 600 Jr manual from MEC's site and ask us if anything isn't clear.