A fork should sag slightly under a rider's weight. The commonly accepted range is for a fork to sag 20-25% of its travel just from the rider sitting on the bike.
Use a zip-tie to help measure sag. Put a zip-tie around one, upper fork stanchion. Snug it down to the bottom against the lower. Sit on the bike. Get on gently, without bouncing, but do put your full weight on the bike. Try to get as close to your riding position as possible. Then carefully dismount. Then measure the amount the zip tie has moved upwards.
BTW, it is common with coil forks to swap out springs to accommodate heavier or lighter riders. Preload dials provide only a small range of adjustment.