Here's a cool starting point formula if your running a tubeless tire - wheel setup on your mountain bike.
Rider weight in pounds divided by 7 = x
x - 1 gives you FRONT tire pressure in PSI
x + 2 gives you REAR tire pressure in PSI
For example my weight is right now 183. 183 divided by 7 =26.1
FRONT pressure: 26 - 1 = 25 PSI
REAR pressure: 26 + 2 = 28 PSI
Of course this is only a starting point to launch from and depending on the terrain, wet conditions, tire tread you can tweek the adjustments