Two inches of rise would be the ideal amount based on the height differences you mentioned. When you can't get an exact number it's best to go a little taller since there would be some suspension sag from the tongue weight of the trailer. The rise/drop figure of a ball mount like the # DTBM4200 is factored with a standard hitch ball so you would not need to further calculate in a balls height as well unless you went with a taller ball.
Now in the case of the Rockerball # C40047 this ball... view full answer...