I don't have an MPP any more, but as you say the conventional wisdom with a 90 mm Angulon is to expect it to cover 4x5" at infinity at f16 or smaller with little or no movement. If you have an abrupt transition from sharp image to total cut-off, this could well indicate vignetting by the lens cone. Remember there is nothing magical about the lens cone, its only function is to allow the front standard to sit back further and be focused by rangefinder with the 90 mm lens. As far as I recall, when I had an MPP Mark VII, I used a 90 mm Angulon with a domed panel I made myself with about 12 mm extension - being a small lens, the Angulon sits in the usual domed panel like a tunnel and could easily vignette with too much tilt. As a rule, using back tilt is safer - it's quite easy to overlook vignetting on the focusing screen in the field.