Mountain Biking - fork PSI

I was always under the impression a fork shouldn't be ran much higher than 150 PSI. I generally run mine at 140 PSI, which is max recommended for my body weight (85% of 160).

I recently measured my sag though and it was 25mm! I'm running my fork between 80-100mm travel, so this is too much, according to the owners manual. I started pumping up the fork until it got to 15mm travel, what my manual recommends for 80mm of travel. I had to fill it up to 210 PSI to do this! This seems like way too much PSI. Is this much OK?

FYI, my fork is an '06 manny nixon platinum. it is really a long travel XC fork, but has infinite travel adjust so I'm running it at 80-100mm, since that matches my bikes geo.

Again, is this kind of PSI normal/acceptable?

I never used or owned a nixon, but 140 psi for 160 lbs body weight sounds too much to me (also too much sag for 140 psi). I'm around 155, and I used to run both my Skareb and R7 at about 70-80 psi, getting about 20 mm sag. There may be a problem with the fork's negative spring or something.

I had an earlier issue (not an issue really, but concern) with this fork related to negative spring. as the fork is set closer to the top of its travel, it gets stiffer, due to the negative spring I'm told. The fork is designed as a 'long travel XC' 'enduro' or 'trail' fork. I think when I run it so low, it will not behave quite right, due to the negative spring being fit for a longer travel fork, or something. actually I don't know how that mechanic of the fork works, so that is just an uneducated theory based on having heard responses on two issues ,of basically the same topic, both going back to the negative spring. makes sense though.