I found they drop about 500 feet after you fire them. Your distances are about right. It depends on your speed (if you're much faster than the target, fire them sooner, as they will fly further).
I did a test in HTH once, I was in a 110G with 4 rockets and had another person in a 110G and I shot then the other shot, we were trying to learn the rockets. So at the same speed(*), the rockets will detonate at 1.4k, having dropped about 500 feet when they do.
(*) Note that if you're faster than the target the rockets will go further, open fire sooner, if you're going head-on try for 2.8k-3k, I've hear others say that works.