Played around with the numbers as suggested, but it looks as if you've already worked-out about the best compromise at .93. The reason I noticed this difference in distance from the margins was because the latest set of photos uploaded had a photo turned long-side up! Strange how it offsets more on up-ended photos than long-side down (if you get what I mean).
I suppose if there was a way of computing the picture width then the arrow could be placed absolute from the margin.
Thanks for assisting me here!