You can take any freely available pattern you buy in a store and convert it to grid form. You can either draw one yourself on ABC pattern paper or take an existing pattern and lay it out. I found this really good book called Wearable Quilts which showed how to do this as the first quilts were actually protection from arrows for warriors. It's got a couple hundred patterns and is inexpensive. I highly recommend purchasing a copy off Amazon
for $.01-$4.00. The SCA also is a great resource for traditional armor patterns. But my favorite is Wearable Quilts.
Once you have your pattern you can figure out how many Pieceables
you need one per 2 square inches in theory. In practice I feel that it is closer to 3 square inches as they overlap around the edges so small pieces use very little to go a long way.