I bought a LHDT this spring, and got it size of the road bike I've ridden for 20k miles over the past 10 years or-so, which is a 54cm Specialized Allez. I had had a professional fitting done before I bought the Allez.
I was able to transfer the fit measurements from the 54cm Allez to the 54cm Surly. I felt was essential as a starting point to get the Surly set up for all the hours I will spend on it. I'm glad I did. I want to adjust a bike to fit my body. I do not want to adjust my body to fit a bike.
Others take a different view, I know. But this was my approach and I'm very satisfied.