Are you referring to the online measurement guide? It can be helpful (for any mask fitting), but I wouldn't go solely with it as it told me some time ago that a mask I already had and liked wouldn't be good for me, and recommended the wrong size in another one. You need to either just get the DME to let you try on (in-house) as many masks in diff. sizes as possible, hopefully while lying down with a pillow, or spring for the return insurance Cpap.com provides to let you try different ones. It helps, of course, to go through all the masks on their site and look hard at different features, but even then it can be misleading as we all have different faces and sleep differently.