You want to make digital negatives, right? Flipping the image horizontally will work. In your testing you want to make sure that the base side of the OHP is always facing toward the light. In some cases printing with the base side facing the light source will give slightly different density on the print than with the ink side facing the light source. I don't understand why this is the case, but it is definitely true for me when making carbon prints using negatives printed on Pictorico and exposed with the Amergraph ULF-28 platemaker.