I look at what size the finished block is and then the size quilt I want to make. You can usually just add another row of blocks top and side to make it bigger. Then I count how many new blocks I would need to make that size and how many blocks are in the original size. It usually is around twice the blocks, sometimes less. If less, you can just double the yardage, if same or more, add more for extra needed for borders. Sometimes, I have cut out the original size except for borders, then figure how much more is needed for the extra blocks, as most times the pattern calls for more than needed.