For the most possible meaningful neck "tension" for any cartridge, first load a dummy round and mic the case neck, then pull the bullet and mic it again. The difference will tell you what the smallest neck diameter needs to be but adjust for spring back by going one thou smaller. Any neck diameter smaller than that simply makes it harder to seat the bullets because the bullet has to expand and stretch the brass; the extra seating force often increases runout.
You may find the 6.5-.284 neck less than one caliper long; there are a lot of cartridges with necks shorter than one caliber.