5 Shot groups and cleaning every 20 is perfect. But youre not completely correct about the pressure corresponding to length. Actually, the deeper the bullet is seated, the more pressure inside the case will increase. The benchrest guys also are not backing off their loads with the bullets touching the lands. In fact, we do the opposite. Once we have length where we want it, we increase the powder load until either the groups start to open up or we see signs of too much pressure: sticky bolts, primers backing out, primers piercing, etc. Your powder load you already have will be perfectly safe with the bullet seated touching the lands. (Ohh yeah, another point, you are seating to the lands, not the grooves.) If you think about it, the bullet has to hit those lands eventually when it's fired. Whether it has to jump or is already touching, the pressure in the chamber wont change. Pressure increases when air volume inside the case decreases. That air volume is decreased either by increasing powder (duh) or seating the bullet deeper.
Now you made an initial. 015 change to find that last group. Go back to that point and try another load. 005 longer and another .005 shorter. Keep powder the same as what you used before. If you find that the .005 longer than before works a little better, then write that down. Then go another. 005 longer than that. Keep taking notes until you see groups open up. When you find a good length that youre happy with, you can change powder load plus or minus a half grain at a time to really fine tune it
If you change ANY of your components, bullet, powder brand, brass, primer, etc, you will have to start over from the beginning.