What is a good dynamic level to practice?

Mezzo forte (medium loud) at the loudest. However, you must never forget why you practice exercises. You do so to set up the correct balance between your exhaled air and your vocal cords, allowing you to sing at a speech level, and to then have your neuromuscular system live with that balance. As far as volume goes, you should only sing as loudly as you are able to maintain your balance with a steady, normal vibrato. The intensity, or loudness, of that tone will come once the muscular coordination to produce pitches freely is established.