I guess this info hasn't been as widely circulated as I thought. There is more evidence that IBUs and bitterness can be derived solely from the dry hop:
http://scottjanish.com/increasing-bitterness-dry-hopping/
http://scottjanish.com/dry-hopping-effect-bitterness-ibu-testing/
http://scottjanish.com/zero-hot-side-hopped-neipa-hplc-testing-sensory-bitterness/
https://hopsteiner.com/wp-content/uploads/2016/09/TQ-53-3-0808-01.pdf
There is definitely an increase in bitterness from using dry hopping. I know this is not the conventional wisdom. I think it may also be possible that tannins or polyphenols in hops add bitterness during the dry hop stage. It is easy to test this out for yourself. Just try adding a very large dry hop charge to a split-batch of beer. you will find that the dry-hopped version has a more bitter, dry flavor. It is very, very smooth though compared with a large kettle hop addition early in the boil. This is why dry hopping alone can be used to provide the bitter edge to a beer, especially if you are doing a beer that is more smooth and soft in bitterness in general, such as NE IPA.
Now, if you like a very pronounced bitterness, dry hopping will not be adequate. I think that can vary based upon the hop varietal, how many beta acids are in the hops and possibly other factors, but in that case you would need to add an early boil addition.
It has been my standard practice to dry hop around day 5 in primary and then leave it for 5 days, chill and then rack to keg. I remember a Brulosophy XBMT that seemed to indicate that 1 or 1.5 days of dry hopping led to a more dank, intense dry hop character compared with a longer one though, so i have been debating adding two charges again, one at day 3-5 and one a day before cold-crashing. I think it was maybe the long vs cold dry hop length XBMT?