so that puts it in the 6 inch drop range...
could be a tad less but if the trailer axles aren't new, 6 is probably good.
i'm at 20 (with the truck FULL) and use ~6...
see this pic (but it's not the same tv or same spring rate)
and here's a member with ~ almost the same tv who switched from 4 to 6 with satisfaction (see posts #211-215)
IF u call their tech support with that measurement
, they should be able to determine this.