I think those that responded already got this, but I'm guessing you are asking about is cartridge overall length (COAL), not bullet length, and how it relates to bullet seating depth. I point this out because there are numerous threads regarding bullet overall length, trimming, and pointing. Clarification might generate better responses from readers.
Every rifle can be different; almost a world unto itself. Within a given caliber, bullet lengths/weights can also vary widely. Additionally, chamber throats often differ between rifles, so that reloaders can optimize a rifle chamber toward a specific bullet length/weight. Thus, there is not necessarily a "universal" COAL measurement that will work optimally across a range of different rifles, even when using the same bullet. Perhaps the most coarse classification of optimal COAL dimensions would whether the loaded round will be single-fed, which can sometimes mean COAL will be much longer, or if it will be fed from a magazine, which places an upper limit on COAL of loaded rounds that will reliably feed from the magazine. From your post, I'm guessing you plan to feed loaded rounds from a magazine, hence the 2.810" measurement, which is basically an upper limit on COAL for .308 Win so that loaded rounds feed properly from the magazine. There can be a little wiggle room on this value depending on the type of magazine system in use, and whether the magazine has been modified to accept slightly longer loaded rounds.
Although it is not perfectly clear exactly what is happening when we adjust seating depth to tune group size, at least in part we are trying to identify some optimal distance relationship between the bullet and the rifling (i.e. the initial contact point between the bullet ogive and the rifling). When loading to mag length, the first step is to determine the maximum COAL that will reliably feed from the magazine of choice. This can be done by making some dummy rounds, or even just taking a simple measurement from the magazine itself. Once the maximum COAL value that will reliably feed from the mag has been determined, the only choice with respect to adjusting seating depth of the bullet is to move it farther down into the case. In other words, you can't seat the bullet farther out in the case neck because the loaded rounds will exceed maximum COAL for the mag.
With a maximum COAL value in hand, one can then begin incrementally seating the bullet deeper into the case, typically from about .003" to .005" per single increment. This process is known as a seating depth test. The idea is to find a seating depth window that optimizes the distance between the bullet and the rifling such that the groups are the tightest (smallest). In the case of mag-fed rounds, you can realistically only test seating depth in one direction (i.e. COAL gets shorter and shorter) as I mentioned above. How many increments need to be used, or how much farther down into the case one can seat the bullet is not well-defined. Certainly there is a limit, as the farther the bullet is seated down into the case, the less effective case volume there will be, and pressure at a given charge weight will increase. With the bullets I use most often, I generally try to cover a total range of up to .020" to .030" initially. However, it may be necessary to move them even farther to find the sweet spot, and a reduction in charge weight may be necessary if one has to seat the bullet much farther down into the case. The bottom line is that you start with maximum COAL for mag feeding, seat the bullet incrementally deeper in loaded rounds, then shoot groups to find out which of the seating depth(s) provide the best grouping.
Due to the restriction of feeding from a magazine, the maximum COAL that reliably feeds will be the starting point, and thus rounds loaded for a seating depth test will only get shorter, never longer. So I won't go into the process of determining COAL with the bullet just "touching" the lands. When loaded to mag length, the bullet may well already be jumping a significant distance before encountering the rifling, dependent on how the rifle chamber is throated. Just be aware that there is another side to such a seating depth test when rounds are loaded for single feeding and thus not limited to mag length.
One final note, although I have used the term "COAL" throughout this response, once maximum COAL for mag feeding has been identified, the process of setting up a seating depth test should be done using cartridge base-to-ogive, or CBTO measurements. The reason for this is that within a given Lot# of bullets, there can be a range bullet overall lengths, largely due to length variance in the nose region of each bullet. CBTO is measured with a caliper insert that seats on the bullet ogive/nose just above the top end of the bearing surface. Presumably, this is very close to the point on the bullet that first contacts the rifling. CBTO is a better measurement than COAL because the caliper insert seats at a specific point of diameter on the bullet just above the bearing surface regardless of how long the nose may be. So CBTO measurements are less affected by nose length variance than COAL. Because bullet nose length within even a single Lot# of bullets might vary by as much as .015" to .020", or more, COAL measurements are generally not very useful for something like a seating depth test, where the .003" to .005" seating depth increments must be highly accurate.
If any of this doesn't register completely, I'm sure your friend can explain everything in greater detail to help you. Having an experienced reloader help you get started rolling your own is absolutely invaluable. Even seemingly simple explanations can be long and difficult to type (and read) at a shooting forum, so answers may not always have the desired level of detail. Working with someone that has experience in reloading can make it much simpler so that the many, many details involved in the reloading process can coalesce into a clear picture. Having a knowledgeable friend help you out is a valuable resource. Best of luck with the journey!
