I'm guessing you're fitting to a full size mattress that is typically 54" x 75". If so, your end result of 400 5" squares appears accurate. However, I think your width should be 88" (54+17+17) and your length should be 92 (75+17). Again, the end result of 400 5" charm squares is the same. If you were to make a top at 71" x 109" it would be a very narrow and long quilt.
Lastly, I never seem to like all of the fabrics in a pre-cut pack so I would opt for at least 1-2 more packs of 40 to allow for a bit of choice.
Hope that's helpful! Can't wait to see what fabrics/pattern you select!
Christine