Yes the answer is the double disk.
The jump on the standard disk would be .46 to .49.
You can use two disk a .23 and a .25 to get .48 or a .27 and a .20 to get .47.
I also use the adjustable bar and can throw 5.7 of true Blue with no variance.
Just purchased a RL550 and to save money I got the basic model. Prime off the press and use the Lee disk and my powder drop. Saved a bunch as I already had the Lee powder dies.