Precisely. In fact, if you have a rack that will fit in the bottom of your pot over the water, use it. It is much easier and quicker than boiling a large pot of water, less messy, and you can grab the lobsters with a towel easily and plunge them into ice water. If you're just steaming lobter all the way to eat, skip the ice water plunge, of course.
Soften the butter on the counter instead of melting it. (I'd use unsalted, btw). To me, rosemary might be a bit much for lobster; perhaps not for you. I'd be inclined to some fresh thyme and parsley with Old Bay and garlic, or a little rosemary with parsley, Old Bay, and garlic. Lots of possibilities though, or try it as written, of course.
One more thing: I'd remove the stomach sac and intestinal tract after splitting, and run skewers between the tail meat and shell to keep the tails from curling when you cook them.