This post is so correct. Provided you're not against the bump stops, the spring will be compressed the exact same amount under load regardless of coil perch setting. There is no "preload". It has to be that way - it's the same spring supporting exactly the same weight, which means the compressed spring length will be the same.
What does change as you move the perch is how far you are from the bump stops. That can lead to perceived ride harshness if you are too close and are hitting the stops frequently. Many people then jump to the conclusion that the harshness is due to spring preload. Not so.