Bowl cozies are Great!
I have made lots of these bowls also. For the size of dart needed, measure and cut a corner off a ready to throw away plastic gift card. Use this to mark your darts. All four darts will be the same size. Also, I use a strip of painters tape to follow when sewing corner to corner. Using tape, I form a 'V' on each side and stitch next to the tape. This gives the bowl a bit of stability.