How about the following:
Have a slope of 1.
Every day, if you don’t have a motif to make and are in a beemergency, add a datapoint of 1. (Think of this as “all caught up for today.”)
Use IFTTT to add a 3 datapoint whenever you enter a datapoint on your reading goal. Now you’re caught up for 3 days but you can’t enter a datapoint until you make the motif.