Beex215,
I wish there was a simple answer to your question, but there isn't. Different chambers and bullets like different seating. At one time, most benchrest shooters used Dahermit's method of kissing the rifling, though most use some number close-to but not quite touching, now. Even they often don't actually know what's best. The Precision Shooting Reloading Guide has the example of a benchrest shooter accidentally turning his seating die micrometer the wrong way when he changed bullets, ending up with them 0.050" off the lands instead of his intended 0.020", and it cut his groups roughly in half. Berger used to recommend their VLD's be loaded to touch the lands, but got so many reports of individuals doing better with them off the lands that they explored this and found their bullets actually did best anywhere from very close to the lands to as far back as 0.165" off the lands (see Berger's letter in the first post
here). It just depends on the chamber and bullet. Item 3. near the top of
this old page is a nice anecdotal example of tuning seating depth producing good results even in a shot out throat.
As to pressure, it does change with bullet seating depth, but not unilaterally. Below is a pressure vs. seating depth curve taken from data in the old U. of Michigan study. What you see is the bullet touching the throat at the left and seating deeper as you go the right. Pressure drops as you do that because the bullet has to jump further to the lands, allowing more gas to bypass it before it obturates the bore. As it goes deeper still, pressure starts to rise again because the deeper seating reduces the amount of space the powder has to burn in, and that finally starts raising pressure enough to overwhelm the gas bypass pressure drop effect.
When pressure changes, barrel time (how long the bullet takes to get out of the gun once the powder starts building pressure) changes, too. If the barrel time was just right to exit the bore at a sweet spot in the phase of the barrel deflection from recoil (often called "vibration" even though the string isn't actually plucked until the bullet clears the muzzle), that will be moved off the sweet spot if you change the pressure enough. That makes it hard to tell if the seating depth or pressure is being tuned when you move the bullet. The only way I know to separate the two is to use a very reduced load when tuning seating depth so barrel vibration doesn't come into it as much. Bullets start to move at about the same start pressure regardless of the charge (within reason - not when you get below about 12,000 psi) so the lighter load doesn't seem to affect the best bullet position much. If you are unsure, retest the best depth with two different powder charges and see if it continues to be the best spot.
Once you have a seating depth established with modest loads that don't rattle the gun a lot, then it's time to tune the powder charge. You'll generally find you need to do it separately for each primer you try. I like Dan Newberry's round robin variant on ladder shooting for this purpose, as it can be used at 100 yards more readily than the old style ladder. You can read about his whole method,
here.