I would send off for sample prints. These would be 4 small versions of the same image (6” x 4” for example) but each file would be slightly different; one would have no adjustments made to it whereas the remaining 3 would have been brightened using Exposure in Lightroom by +0.10, +0.20 and +0.30 and then exported out with the following files names: test, test+1, test+2, test+3. When these prints came back from the Lab I would then compare them to the image on screen. The one that matched the nearest would then tell me what changes I needed to make before sending the main file to the Lab...does that make sense?