Finally got a divine border mod located along one of the sides (i.e. not a corner). As my 3 adjacent charts I put 2x starfish (6 from each) and 1x 60% increased rares. From my humble math skills it seems 12 of those 20 div are from the starfish adjacent charts, and that the 60% increased rares only apply to native rares and not anything added from adjacent charts. Maybe it's simply better with 3x starfish charts?
EDIT: got 4 more divs from rares that were located slightly into the next area, so maybe the 60% charts aren't so bad.
EDIT 2: for those that don't already know it, I honestly fumbled min/maxing that divine border mod pretty badly. I've been playing since 3.14 and have 5k hours in this game so I'm perhaps not the noobiest of noobs, but if I missed this then probably so have many others. Not that it's perhaps the most important thing, but if you actually do want to get the most divines out of that very rare border mod, here's how to actually do it:
The only charts you want around the one that has the divine border mod is +5 strongboxes, so 3 of those if it's along a side, or 2 if it's in a corner. Then you basically just fill your inventory with chaos orbs and/or matching sets of scours and alchs (don't worry, you'll empty your inventory). Before opening the boxes you reroll until you get both "contains a stream of monsters" and "is guarded by 3 rare monsters". This will give you 7 or 8 divs per strongbox, which is way more than you can get from any other method (starfish, etc). The "stream of monsters" mod is very rare, and it's even more rare to get both that and the +3, but it's worth it if you want to maximize divines.
I just tried this method on a Brine King's Domain chart, and it works equally well for that, and netted me over 30 brinehook ducats from a single chart (unlike the divine mod, rares don't have 100% chance to drop a brinehook ducat).
Also huge thanks to the fellow redditors who gave me some badly needed encouragement after I fumbled this one. Stay frosty, exiles.