Besides the KDX the most modern bike I have is an 81 YZ465, and the suspension was perfect when I got it, previous owner spent some money there, and we are about the same size.
No such luck with the KDX. Ive ridden it a few times, first time out it wasnt so bad, but then I adjusted the collars a little in the rear... last time it almost F***ed me. Was at a practice day at an MX track and going across some ruts, the bike handled badly, got out of shape and I hurt myself a little hanging on to it. Also the rear fender kissed the rear wheel a couple times...

I got the 5.8 kg/mm racetech spring on the back, and just set the sag. Right now Im sitting at 105mm with me on it, and 35mm without me. Collars are dead in the middle of the range. Id say racetechs calculator is dead on. Made some initial settings on the clickers.
Good enough for me. To be honest Im not that great a rider, I just dont want the bike to kill me.
Im going to add some spacers and heavier fork oil now. Might do some different springs at some point. I think the front is what messed me up over those ruts.
Ive got a good test for it tomorrow, a vintage cross country race. We'll see if I can make at least a lap or two. Im fairly new to the woods, I tried a few CCs but on bikes that were NOT well suited for it. Really excited to see how the KDX does tomorrow!!