After riding and thinking for a few days, and getting really annoyed at that 11-tooth cog taking up space, I finally found the time to dig up an old but virgin 30-tooth cog from a seven-speed cassette we'd dismembered several years ago.
I like the steps in my 13-28. It made more sense to add the 30 to give me a 30-30 low on the granny ring and let me ride the 48-28 in the next-to-Ned.
The next-to-Ned is the gear just below the Ned gear. The Ned gear is the full-cross, big chainring-biggest cog combo. Most bike shops advise new riders to avoid full crossover gears, big to big or small to small, for a variety of good reasons. Mountain biking legend Ned Overend stated, in a magazine article some years ago, that he loves that gear and uses it often. If you read carefully, you found out he doesn't cruise in it, he just uses it for short stretches when he knows he'll be shifting back down into less forbidden gears from the big ring. Using the Ned gear often avoids having to make two double shifts to find the approved mid-range gear and then return to the big ring.
I could never get over my guilt in the Ned gear. I was raised a Puritan, when freewheels had five cogs and you never rode crossovers.
All this moving of cogs has messed with my muscle memory a little. Shifting in friction, you get used to feeling the lever at a certain angle for a certain gear. When you move the cogs, the shifter has to stop in a different place. It's like re-tuning a fretless instrument. The familiar finger position won't produce the same tone. But I'm getting used to it.
The last step, from 28 to 30, is a bit weird, since the cogs go 21-24-28-30. The jumps go 3-4-2. At some point I might dredge up a 32. But 30 is low enough, so it would just be to satisfy some mathematical neurosis.