The ST1300 is sprung for a 150 lb rider. There is no preload setting on the stock shock and spring that is optimal for more than double that weight. The best you can do short of a new spring is to ensure the preload mechanism is working to 100% of designed travel of 10 mm at 36 clicks and use a lot of it, probably all of it for 330 lbs, to control sag. All of it.
The rear shock preload mechanism loses travel and effectiveness over time but the fluid is easily replaced to restore 100% travel and is considered essential maintenance on ST1300s with any miles. There are many posts on this and it's an easy DIY job. The preload wheel turns clockwise to add preload. From the soft setting fully CCW it turns 18 full turns CW to full hard. If the wheel turns CW without resistance the preloader is not working. When it starts encountering resistance it has begun adding preload.
Damping is set by the set screw in the body of the shock. Standard setting is 1 turn in from fully CCW out. On the stock shock I liked settings of more than that - from 1 1/8 turns to 1 3/4 turns. With extra weight above the 150 standard you will need more damping.
Good luck and great riding!
https://www.st-owners.com/forums/content.php?269-ST1300-Pre-load-Fluid-Replacement
https://www.youtube.com/watch?v=CAhksxRCIik&t=11s