Here's a definition of the 'ideal bullet position' from Robert Whitley's description of his 6mm / 6.5mm SLR designs.
http://6mmar.com/Super_LR.html
Note: When the term "Ideal OAL" is used, it means the junction of the boat tail and bearing surface of the bullet is forward of the junction of the neck and shoulder of the case by about .025" or so. When the term "Forward in neck but good OAL" is used it means the junction of the boat tail and bearing surface of the bullet is forward of the junction of the neck and shoulder of the case by about .040" - .070" or so (depending on the bullet involved) which is still good because the cartridge neck is .322" which means you have about .250" plus of neck still holding onto the bullet.
The 6SLR has a very long (0.322") neck giving lots of (seating) room to play with. The 6.5mm Creedmoor is better than many other off the shelf designs, but is shorter at 0.259" if I've done the sums right off a cartridge drawing.
So, it's not the base position that's critical for you, rather the bullet shank-to-tail section junction. (Assuming you're not loading flat-base bullets where it is the base that counts.) You want that point on the bullet to be just above the case neck-to-shoulder junction - as RW says, 0.025" above is a good place.
You can ascertain this two ways. The quick and dirty way is to seat a bullet gradually deeper in a sized but unprimed/uncharged case and put the same model of bullet alongside on the bench with the tips level to see where the shank/shoulder junction lies in the case and judge that ~0.025" gap by eye using calipers set at that value to give a guide. When you're there, then measure the resulting COAL and see how that matches your magazine and/or rifle chamber freebore.
The other (more scientific) way is get a drawing of the bullet - as in this drawing on Berger Bullets' website of its new 156gn EOL Hunter:
https://bergerbullets.com/new-berger-6-5mm-156-grain-eol-elite-hunter/
and see where the tail to shank junction lies - ie take the quoted OAL and subtract the tail length, 1.512 - 0.189 = 1.323" from the junction to bullet meplat (tip).
Then take that value and subtract the case-neck length. (But first measure the OAL of the case you're using - you'll need it later.)
1.323 - 0.259 = 1.064" for the Creedmoor. That amount of bullet should be visible outside of the case, but we want that 25 thou' of clearance between the two junctions so add that back in to the bullet outside of the case-mouth:
1.064 + 0.025 = 1.089"
You then take that value and add it to the case OAL. The 6.5mm Creedmoor at its maximum SAAMI length before trimming is 1.920". so let's assume your case is exactly that. Add this figure to the calculated one for the bullet to get the cartridge OAL (COAL) for that bullet seated at the optimum position in a Creedmoor case.
1.920 + 1.089 = 3.009" ........... over SAAMI max COAL but no surprise for this very long / heavy bullet.
You then have to reconcile this to the rifle chamber and / or the maximum COAL that the magazine will feed reliably.
............ or if you've settled on this model of bullet and are willing to single-load it, you make up an inert round, take it to your gunsmith and get him to ream the throat / leade so that the bullet just kisses the lands (or is 20 thou' off or whatever you want).
........... or if the magazine length and resulting COAL is the primary determinant, you either don't load that bullet and choose a shorter model. or you stick to the long model and live with that COAL and seat the bullet deeper to suit the magazine accepting it's a non-optimum position and that the bullet shank and tail combined sitting below the case-neck may take up a great deal of case capacity that you want to put powder in. You also still need to check how that COAL and bullet shape match the chamber freebore if the rifle has already been chambered (or tell your gunsmith to throat the chamber for that bullet at that COAL giving him an inert example).
The 6.5mm Creedmoor chamber was designed to see the 140gn Hornady AMax more or less ideally seated in the case at 2.825" COAL. Some of the tail will be below the neck, but that's OK as noted. Longer bullets will be seated deeper though, but if your magazine feeds reliably at the 2.870" you quote, they could still be fine.
As the Sierra 150gn MK is one of the new super-long match models from that company and your chamber was likely set for a shorter, blunter 140gn model, it may well be seated deeper than ideal at 3.183" (less any jump you want to build in). Depending on the powder used, the 6.5-284 case usually has plenty of room, often quite a bit spare in fact. Providing no 'doughnut' (internal ring at the neck-shoulder junction) turns up, you'll probably suffer no issues from a bit deeper seating that sees some bullet shank below the neck.
The other answer is as before - seat a bullet optimally in an uncharged sized case and take it and the rifle to your gunsmith and ask for the chamber to be rethroated out a bit tgo suit. (A simple job using a hand throating reamer that's done with the barrel in situ in the action. Remember though, that whilst you can always take more metal out, you can't replace it. If the 150gn SMK at that COAL doesn't perform for you you're stuck with the freebore for it and shorter bullet models may end up with a great deal of jump to the lands even when seated relatively shallow in the case neck.
......... that's why there are so many threads on this forum asking about individual bullet models by make / type / weight / calibre, (not to mention optimum rifling pitch rates - another large can of worms) especially as the further bullets are developed to reduce drag and the longer they become in relation to calibre, the more particular they usually are. So, the 'Search' facility on this forum is your friend. There have been many such questions and multiple responses on the 6.5mm 150gn SMK for instance since its introduction 18 months or a couple of years ago.
