Just to make sure the dirt simple solution doesn’t work: If you know you’ll need at least 250 (or at the very least 200, or some such number like that) then what about just beeminding it the old-fashioned way? Straight up do-more where you add a +1 when the motif makes it onto the blanket. Give it a week or a month of safety buffer to start with to avoid having a blanket beemergency without the corresponding book having been read. If the book reading is beeminded too, then that should mostly work. Although I agree it would be super cool to have the goal automatically dependent on the book reading.
This also reminds me of fractional beeminding, as @bee describes here:
(Fractional beeminding is also recapped in this delightful old blog post on bucketminding)