Well, REW uses relative time, so you do indeed need a reference as you've said already - (actually the next release of REW is rumoured to have a loopback on the other channel to solve that problem, so you might want to wait a bit).
I found it a bit difficult to identify peaks from reflections, and certainly when doing one of the most important distance setups that is between the sub and mains, it becomes very tough. The sub has an extremely broad peak due to its limited bandwidth, so it can be hard to see the individual peaks of the sub and main on a single plot.
My telling you won't convince you that it's tough - you'll have to try it once and you'll see what I mean.
brucek