bonus bAtkRate, (MaxHp * percentage_of_maxhp) / 100;
This will increment the ATK by the specified percentage of the max hp. I'm sure this is not what you want, but it is what you requested.
If MaxHp is 500, and percentage is 1, then this will add a bonus of 500 * 1 / 100 = 5% to ATK. If you find that this is too much (you probably will), you can multiply the denominator by a constant which better suits your needs.
In my opinion, this will never be an interesting bonus, unless you make it non-linear, so that the bonus gets smaller as the MaxHp increases.