Good points as far as the distance from mouth to mic. Also important is technique. It is important that as you sing you keep that distance consistent or you will be doing gain leveling across sections of the track. I speak from experience. When I do a reference vocal track I do it sitting at the keyboard holding the mic. That can make for variation of up to an inch here and there and it makes the levels go up and down like a roller coaster. When I do the real vocal take I am in a booth and there is nowhere to move so I am able to keep a consistent distance. Make sure as you test mics you apply all of the above. It is great advice to have another set of ears test you natural voice vs the way it sounds coming out of the mic.