I'm riding a Lehman Raider (relatively light) trike with a curb weight of 833 lbs. I'm 230 lbs, for a total of 1,060 lbs. My rear tires are 205 / 60 R 15" (R = radial). Mine came with extended control mini-floor boards, which moved my weight distribution directly into my backbone. The Harley Sundowner™ Seat was soft, and wider than stock, but that didn't help on bumps and asphalt patches (and Pennsylvania is the pot-hole patch capital of the world).
I always ride alone. After changing my (OEM) shock absorbers to soft, I changed my rear tubeless tire pressure to 16 lbs. After riding it around the neighborhood (to make sure I didn't get a flat by under-inflating the side walls) a found it was a little too soft ~ my Sportster front end wandered / wobbled a little on the turnpike.
I changed it to 18 lbs, that seemed about *right*
. I have a 1 - 20 PSI pencil gauge I keep on the trike. I've been riding for a few months (again, 1-up riding) and it seems to be the best compromise.
The front tire pressure is 38 lbs, which has little impact on the ride performance.
In summation, should *you* try 18 PSI? = Not necessarily. Should *you* experiment? = YES