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 "recommended" 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.

This is not really an answer but a long comment.

The trick word is "optimal". Optimal for what?

For an MTB rider, where riding conditions are extremely dynamic, weight distribution will change a lot during a ride.

One such variable is terrain incline. While ascending weight is shifted towards the rear wheel. Descending is the opposite.

Other variable is whether you are braking, as it shifts weight towards the front.

Depending on your riding technique you can intentionally shift your weight in any direction to further assist your maneuvers. Getting out of the saddle gives a wider range of motion.

Specifically on getting traction in the front wheel I have found that:

1. For cornering a tire with tall knobs is better, if those knobs are elongated and placed more longitudinally (in the direction of tire rotation), even better. A good tire with a good thread pattern and good compound saves a lot of pain.

2. While descending I lower my torso to have a lower center of mass, which allows for a slightly more weight being applied to the front tire while diminishing the possibility of being thrown over the bars. (I can brake and corner harder while doing this than while not).

3. For cornering either on flat or descending, slightly bend your elbows outward and move your chest a little towards the handlebar. This puts a little more weight on the front tire and also puts you more into the "attack position" where the arms are more ready to react in case of slippage, some times even enough to recover promptly, re gaining traction in a fraction of a second. This also makes your arms assist the suspension. It's specially useful when using a stiff suspension or a rigid fork.

For both 3 and 4 (which are almost the same thing) I'ts often necessary to move the butt out of the saddle towards the back to compensate. Both pedals should be at the same height and both knees slightly bent.

4. Brake balance: In some parts of the trail I apply the rear brake not for speed reduction, but to put more weight into the front tire, increasing how much friction it can provide. When going over slippery stuff (a wet tree root, mud patch, etc) with the front tire I release completely the front brake until I'm again over more favorable terrain.