I was not able to download the pattern - but I was thinking/wondering -
After cutting the two 2:5 x 21 inch strips -
If you made the next cut 8.5 x 21 inches - then cut that piece into 8.5 x2.5 inc pieces.
Theoretically, there should be a strip 4.5 x21 inches left.
If your FQs were wonky, or skimpy, then there will not be that much left.