Okay, you have two questions.
First, max loads for bullets you don't have data for. Don't sweat the the small stuff. Any difference between what you have and what the books specify for bullets of the same weight (including the specific case, powder lot, primer, etc the book makers used) will be accomidated by THE SINGLE CARDINAL RULE of reloading; "start low (in charge weight) and only work up (in small increments, maybe .2 or .3 gr. depending on case size) until you reach the book maximum OR until you see evidence of excess pressure. IF/WHEN you do see pressure signs, back off 1 or 2 gr, depending on case size, and accept that as your own max charge for that bullet in YOUR rifle, no matter what the book says. If we don't do that, we could load for a precious few bullets and could even KABOOM a few guns with "modest book loads!"
Excess pressure signs you can't miss are (1) the bolt is "sticky" to open, (2) you see a shiney spot on the case head where the ejector cut has sheared off a bit of the brass, and/or (3) you see a smoky ring around the primer pockets. ANY ONE of these is a warning to back off, NOW! And forget studying "flattened" primers as any real help, there are just too many other things that flatten primers even if the pressure is normal. If you do get an over-pressure flattened primer one of the other three signs will confirm it, no matter what the primer looks like.
Now, the OAL length. Understand that the book OAL is no more of a "law" than the powder charges they list. It's ONLY the length they used to develop their data, so any changes you make in OAL
will be accomidated with the same process and signs used to watch for excess pressure working with charges.
For rifles (not handguns), seating in the lands has a risk of increasing pressure, but not when backing off and seating deeper. Well, at least within reason, we don't want our bullets to fall inside the case you know! Therefore, I think it's easiest - safest really - to start load development with the bullets close to the lands - maybe off .010" - and, when the best charge is found go further from the lands until you find the best seating depth for accuracy.
It is NOT true that factory sporting rifles tend shoot their best into, or even really close to, the lands. That's a myth coming from BR shooters. But, they ain't us. BR shooters use rifles vastly different from ours so they use different cases, chambers, bullets and loading methods. What works "best" for them isn't consistant with us and rarely works best for us. In fact, their methods are as different from ours as tennis is from badmiton; looks somewhat alike but not much!
MOST factory sporters seem to shoot best with bullets from .020" to as much as .100" off the lands. Meaning I'm NOT saying there aren't any exceptions, both ways, but most of us will find better accuracy well off the lands. Only way for us to find our own best seating spot is to experiment, same process as with powder, while looking for groups instead of pressure/velocity this time. Since backing off the lands IS safer than moving closer, it's best to start close and back off after you've established your max charge. Seems tweaking the charge a little after finding best seating depth would be good, and it may help sometimes, but it's never done a thing for me.
While doing these tests, as you shoot and reload to shoot again, keep a close watch on your case length. If any case exceeds the chamber length and a bullets is gripped in the chamber rather than being free to move, chamber pressure will instantly go outta sight! It's easy for a newbie to forget to watch for this, don't you do that!
Oh, yeah. One more "question"; How do you determine what's "best" for powder charge or OAL? You will find, or should find, that the actual charge and OAL that works "best" isn't a tiny, critical spot, it's usually within a power range of maybe .4 grains and an OAL range of as much as .015". If your load is more critical than that it will also be quirky, tending to do crazy things with tiny changes in temp, or powder, primer and bullet lots, etc. That shouldn't happen. A good load will be surprisingly tolerant of small differences without notice to the shooter. Meaning, weigh your charges until you find the load, or OAL, then load in the middle of the range that works but after it's found don't sweat slight differences. Do this and many otherwise "unexplainable fliers", will disappear.