Underexposed or not, that's a neat shot. I'd bet another half stop should do it.
That data sheet on the 100G strikes me as a bit odd. They say no compensation necessary for exposures up to 10 seconds. Then they mention some color correction at 120 seconds, but unless I missed something (it's happened!) there doesn't appear to be any exposure recommendation for the range from 10 to 120 seconds. Surely it would likely need more than color correction.
There have been some discussions on this forum sometime back about reciprocity failure in B&W films that seemed to suggest the makers' indicated numbers were over compensated, and that maybe the manufacturers are running with ancient numbers. That could be another reason to do your own tests. I'm pretty convinced the exposure numbers from Pinhole Designer for Delta 100 were overcooked, based on my limited experience.