The 7G RMW recommendation is consistent with your 8D foot. Sizing up in length to 7.5 while sizing down in width to F often works for people. Some opt for this adjustment in an effort to retain a good fit while achieving a sleeker look. 8G will very likely be too long and too wide for an 8D foot. That there is pretty much how it stands. I don't think you can get much more sizing info without getting your actual feet into some actual boots and seeing what works.