Half your normal PSI should be a good place to start.
Check the door sticker and work from there. That said being conservative is definitely a good idea if you don't have an easy way to swap to the spare if you did slip a bead.
On sand it's your call. I always air down at the start of a trail. If you're fully inflated you're much more likely to dig yourself in and get stuck, whereas if you air down you're less likely to get yourself in a situation where you're relying on the maxtrax, shovel, CRAWL, etc. I'd rather think "that was so easy I'm not sure I needed to air down" then to be outside sweating through (or during) a recovery. If this was rocks or a forest trail I'd say deflate at the start because the pointy stuff can tear a sidewall if you're fully inflated since the tire can't deflect, but sand is soft so that won't be your issue.
From the internetz on taller 17s or 18s it seems like 15 is reasonable, though with a shorter tire profile it will depend a bit:
The Aussies say run 16 psi in sand, and temporarily drop it if you get stuck:
The best tyre pressure for sand depends on a few things, but most people don’t let nearly enough out. Find out more here.
www.4wdingaustralia.com
and
Setting your tyres to the right pressure for sand can mean the difference between a great day and a nightmare scenario.
www.expeditionaustralia.com.au
.
"By letting your tyres down to 16psi you significantly increase the size of the tyres footprint on the sand so you have the same weight of vehicle spread over about twice as much rubber and therefore half the weight per square inch.
...
You can go as low as 6 or 8psi if you need to but only to get you out of the s*** - make sure you have your compressor handy and pump them back up to 16psi again when you're clear and back to road pressure before you head home."