Give this link a go. I found that it explains things very well. I've used this method many times with good results.
http://www.racetech.com/articles/Sus...AndSprings.htm
I've just gone through the process of setting the pre load on my 05 ST to 35mm, both ends. I'm 6'0 at about 75 - 80kgs and found that I nearly ran out of adjustment increasing the pre load. With only 12,500kms on the bike, this means the springs are on the soft side. With rebound only damping on the rear, the bike has a hard time on sharp bumps, resulting in a harsh ride.
If I load up to go on a trip, or carry a pillion, or both, I know the soft springs and lack of damping arent going to cope to well, so taking it easy will be the order of the day.
I had the same problem on my '99 ST. Upgrading the suspension is money well spent. Why Triumph didnt put fully adjustable suspenders on the 05 & 06 ST, or at least make it an option, is beyond me.
I mucked around with the suspension on the '99 ST a lot. This time I'm saving to fully upgrade at both ends. It will be on the expensive side but worth it. So far I havent heard of anyone doing this on this model so when I do I'll post my results.
Cheers,
Brett.