Take the chain off first.

It's real easy to see how the rear sprockets line up with the jockey pulleys when the chain is off.

Of course, you'll want to test with the chain on when you're done.

unnecessary and inefficient, sorry

I always set up a new RD with the chain off.
If it is Shimano, it often requires no further adjustment after installing the chain.
Campy generally requires more fussing.

A solution without a problem.

With the chain ON, you can also see how the chain lines up in the cage, which is more important.

(NOTE: When you're REALLY good, you don't have to look at the derailleur at all to tune it -- you can do it by EAR! Sadly, I've lost that fine edge....)

Very efficient if you already had your chain off for cleaning like I did. Would have never occurred to me otherwise. Trust me, it's much easier to see how things are lining up without the chain. I had just gotten a new cassette (different brand) last week. I did a quick adjustment when I put it on because it didn't quite match the old cassette. Was surprised to see how far off it actually was.

The Campy instruction booklet I have recommends doing the adjustment with the chain off, so that's the way I do it. The top pulley has a little play that can interfere with my assessment of the alignment with the chain on, so it ends up being faster for me to just remove the chain.

Good to know that I'm not being an idiot. If you've got to break out the chain tool it's probably not worth it but if you've got a power link (or equivalent) you might as well give your chain a good cleaning while you make your adjustments.

Still seems like a bunch of unnecessary work. I've never had to do this before and I've adjusted lots of rear derailleurs. When you move your chain into a cog and it's not lined up correctly, you'll know pretty quickly. Same for high/low adjustment. If you have adjusted the limiter screws too far, the chain wants to pop off the cassette.

I don't bother lining up the RD with the sprockets. I just put the thing together and run it, and then adjust until it shifts right. This typically takes about 3 minutes total. Then I take it for a ride and dial in with the fine adjustment on the shifter. This takes another 3 minutes.

I've haven't had to do it before either, it just took some of the trial and error out of the process. Normally I'd start with the chain on one of the center cogs and attempt to center the chain between the nearby cogs. Then I'd run through the other gears loosening or tightening the cable as necessary to get clean shifts.

Without the chain it's like lining up two pointers. Next time you have your chain off, take a look and see what I mean. It's probably more helpful when you're dealing with more sprockets and less margin of error.

+1

This has worked for me for years

Seems like a good thing to take a look at if I've got the chain off already - I usually just adjust until the shifting is right, but if I'm already at the RD threading the chain back through after cleaning it, why not spend fifteen seconds twiddling the adjuster to forestall any upcoming shifting problems?