I prefer setting preload for effect, rather than by the numbers. I like to preload using about 1/4 of the travel, so I have 3/4ths left and room for over travel on rebound.

You'll need a twist tie, pipe cleaner, or similar.

Set the preload at some logical starting place. Tie the twist tie at the base of the sliding part. Ride a few seconds on smooth pavement, then see how far up the tie has moved. Adjust preload accordingly.

Reset the tie, and leave the there for a number of rides. Ideally, it will get pushed almost to the top, meaning you're using the full travel range without bottoming out.

If you tend to bottom, use a stiffer elastomer, if not getting much travel, use a softer one.