The way ASCE words the definition of mean roof height, if you have a flat roof then the mean roof height is the height of the eave above the grade. So in your case it seems to me that your choices are to use 20 feet, 50 feet, or something in between. I wouldn't go with 20 feet since that would be the least conservative option. If it was me I'd just say 50 feet is the mean roof height. Would using 50 feet instead of, say, 35 feet change the design very much?