Enough lurking!
Last year my ACCs were wearing out (aluminium starting to show near the point !) and as I suspected that my 3-28 (500) were too stiff, I bought a couple of 3-18 (560) and another couple of 3L-18 (620) to experiment with over a couple of weeks.
With the rear of the bare shafts taped up to match the weight of the impulse vanes I was using at the time, at 30m I found that whilst the 560s were consistently fairly close to the central fletched shafts, the 500s were about 10" from the left hand edge of the boss (RH archer), and the 620s
occasionally hit the right hand edge of the boss, but most were in the green
This was typical, but wasn't totally repeatable across several sessions - sometimes the differences were significantly smaller, and the variability must obviously have been down to the pink bit at the back, and the unforgiving nature of bareshafts. There were actually sessions where I wondered if the 620s might be an option, but others where I could barely keep them on the boss, however the 560s were consistently good whatever I did.
When I fletched all three spines, there wasn't much difference to be seen.
Out of interest, I did experiment with taped vs untaped on the 500 bare shafts, and the difference (for me) was typically around 4 to 5 inches with the taped showing a little stiffer as you might expect.
In the end as the 560s seemed consistently good, I went with 540 ACGs with XS vanes (slight rear weight reduction and weakening effect compared with the impulses), and with a slight tweak of the limb bolts I'm really happy with how they fly (most of the time)
So
for me last season, one spine seemed to equate to around half a boss at 30m.
From my experience, bareshafts seem to be
very sensitive to quality of the shot - I've just updated my riser and did a bit of bareshaft tweaking last week, and whilst well-shot bareshafts are now in the group, slightly poor quality shots can be an easy six inches out of the group - for reference I shoot at a fairly consistent Bowman standard.
With the new riser, my bareshafts initially were probably showing somewhere around 30 cm stiff, but after winding in the limb bolts to give around another lb,
well-shot bareshafts are now nicely in the group.
If you think your shafts might be too stiff and the budget will run to it, it might be worth getting a couple from the next spine weaker to play with before you bite the bullet - I'm glad I did.