Another Merlin newbie...
 

...with an HV20.

I found the system fairly easy to set up, but I'm finding it difficult to keep the roll-axis trimmed (ie keep the horizon level). Any tips?

Well, unfortunately that is the trade-off of such a light camera--very low inertia. With practice, your fingers will "learn" the right amount of touch to reduce the tendency for the rig to roll without overpowering the stabilization. If it makes you feel any better--my first time with the HV20 on the Merlin delivered not-so-great results, and that is on top of 20 years flying the big rig! The HV20 will fly better with more weight added, such as the Bogen quick release base (search the forum to see discussion of same), or its own accessories like the H43 wide angle adaptor.

Charles, thanks.

It isn't so much that the rig has a tendency to roll when I move it, but rather that it always seems to want to hang crooked one way or the other, even at rest. I've looked at the magic formula spreadsheet, which gives workable solutions for all three nose-weights, but I'm still having trouble.

Will the quick release plate help? I'm assuming that the reason for this problem is that with the viewfinder open the camera is much wider than the rest of the rig, so the most effective solution would be to add some form of extra weight to the camera side of the gimbal which is as narrow as possible. Does that make sense, at all?

Sounds like you're still top heavy...do a z trim to lower the weights to fix it

Yes, agreed, check your drop time and make sure you are not neutral-to-top-heavy.

Don't worry about the offset of the viewfinder, as long as you make the side-to-side adjustment to rebalance for that you will be OK. The viewfinder itself isn't that heavy. It's quite normal for the camera to be wider than the rig itself, in fact it's preferable as that will add needed inertia into the roll axis.

Guys, thanks.

It's definitely not top-heaviness. I think you had it right with your first answer, Charles, it's just that the camera is so light. I tried a wide converter on the front and the added weight pretty much solved the problem.

The main drawback is that I had to add a middle weight to the lower spar and adjust the arc size so it's not something that you can really pop on and off between candid shots. I guess the best solution would be to add something of similar weight to the front of the stage when you take the wide converter off.