With shorter strips you just keep halving the number 100 until it fits the page number of the story you want to illustrate, ie: 100+ pages on a story, you illustrate page 100, less than 100 pages and you illustrate page 50. Less than 50 pages and you illustrate page 25, less than 25 and you illustrate page 12/13, and so on. If you're doing Al Ewing's Future Shorts - I have no idea.
I think the best approach if you're stumped isn't to pick a different story and hope you land on a page with a zombie massacre, but to capture dramatic tension between characters, foreshadow events yet to happen, reference past events, or take the most dramatic incident on the page and stage it as dynamically as you can.
Possibly it would help if you treat it like you've been asked to draw a cover image for a strip that just happens to be entirely exposition that week, like that Dredd cover where he's screaming "I quit!" and waving his badge like a looney, yet the strip behind that cover was an introspective piece with no action.