I have a 26 gallon bowfront and use a medium magfloat and i do use it vertically and i have no problems getting the glass completely spotless so i imagine for a large tank you will have no problems. I thought that it wouldn't work well, but it's perfect.
I used to do it about once every 2 weeks, but it's been easily over a month and the glass is spotless, so i got my dosing and lighting down.. now i'm just worried there isn't goin to be enought algae in the tank for some of my fish!
EDIT: I didn't realize this thread is an old thread! sorry, lol