Optimal front / rear weight ratio
I have been experimenting with tire pressure and stem length trying to improve front tire grip and steering on my trail bike which is often slipping in corners.

I cannot see how tire pressure can affect weight distribution in any measurable manner. A very minor effect on the static weight distribution may perhaps be observed on a fat bike, where a severe underinflation of one tire actually changes bike's geometry (by visibly raising or lowering one of its ends). Adjusting tires' pressure may help with getting a better traction, but the involved physics do not have much with the weight distribution to do.
Changing stem length is obviously a different story, as a longer stem may force the rider to shift their center of mass forward.

I'm guessing I might get closer to 45/55 on a road frame. But I am nowhere near that on my mountain bikes

I do not see how a &quot;recommended&quot; weight distribution, if there was such a thing at all, provided for a road bike can be applied as a measure of anything for a mountain bike. These types of bicycles have different dynamics and areas of use.
If we track trends in changes of static weight distribution for mountain bikes over the last three decades, I am sure that we will see a trend of weight shifting from front to rear. MTBs of 90's were inspired by road frames, and modern MTB's tend to have rather short chainstays.
What really makes these numbers irrelevant is the fact is that under braking/acceleration the weight distribution between front/rear wheels is nowhere close to the static values.