I think you need only 6 petals/block, plus extras for the edges (assuming the edges are finished with petals. If I'm right, you'll have:
6 petals/square foot x 49 square feet = 294 petals for main blocks
plus
2 petals for each of 4 corner blocks = 8 petals
plus
1 petal for each non-corner edge block = 20 petals (5 edge blocks/side x 4 sides) =
322 petals
This said, please understand that I have never done a quilt like this, so I could be entirely wrong. I hope others will chime in, too.
One more observation: You say these are 12" blocks, yet the instructions say they are 11 1/2". Did you use a compass length of 6" to draw your petals, instead of the 5 1/2" recommended? I ask only to alert you to the fact that your final result might not be quite queen size if you used the 5 1/2".