Keep in mind that if it is 'Other', it can easily come out on turn 1 or 2 unless it's quite expensive.
If you don't divide by 2 after taking the square root and don't add the 3 (making the formula just ATK = square root of your HP), it would have 20 ATK based on 400 life (the rest of the chart would be 16, 12, 10, 8, 6, 2. It was an example, but square root is necessary for diminishing returns, and I had the basic attack be 3 so it wasn't too weak if you're nearly dead.
I would personally like it if it fluctuated with your HP at the end of the turn, but either way works.