To replicate pan film with ortho, use a green filter to darken the sky. If you dont use a filter, the sky will print out as white with no cloud distinction. Red, orange, and yellow filters have little to no effect. Underexposing ortho film won't gain you any advantages, just underexposed film. I've shot two boxes of Ortho+ 4x5 so far and it does not have the exposure latitude of pan films.