UPDATE:
Since my initial test a couple weeks ago, the thought occurred to me that the POI I got that was higher than I expected was probably caused...at least in part...by shooting off a bench rest.
Back to the range today to retest from my normal seated hunting position, leaning back against a post like it was a tree, off elbow braced down on my chest...got a noticeable improvement in POI.
Tried 100grns at 50yds again, and while the double PRBs printed lower than a couple weeks ago due to not shooting from a bench rest this time, they were still a bit above the 3" aim point sticker, higher than I wanted at 50yds.
Tried shots using 90grns Goex 2F (which I had skipped last trip) and got figure-8's in the top half of the 3" sticker...basically 1.0" to 1.25" high at 50yds which is just fine.
2 pairs (4 balls) actually created one large ragged hole in the same place...the other pairs made separate figure 8's, resulting in all 5 pairs being in a 2.0"-2.25" cluster at 50yds.
So 90grns 2F it is, for some extra velocity compared to the 80grn charge I tried 2 weeks ago.
Very high confidence in this experimental .45cal double PRB deer load.
Closing note of Irony:
My first reaction to the accuracy of these double PRBs was "great"...who WOULDN’T think figure 8’s at 50yds was great.
But then the thought occurred to me...they would probably be more effective on game if the POI actually had a 3-4" separation...but, it is what it is.