Now that I think about it, this is vaguely similar to another card in the master list:
Hope Amid Suffering
{W}{W}
Sorcery
If an opponent has at least three more life than you, you may draw two cards. If an opponent has at least seven more life than you, you may draw four cards instead.That card looks for fixed life differences and yields fixed card advantage. It's also a sorcery -- if you want this to be an instant, it's going to raise the cost significantly. Looking at the ratios of life difference to card drawing, though, it would probably be easy to get away with another card like this:
Brains over Brawn
{W}{U}
Instant
If an opponent has more life than you, draw cards equal to half the difference, rounded down.Up to a difference of eight life, this yields similar card advantage to Hope Amid Suffering, with a little more granularity. For life differences greater than eight life, this yields significantly more card advantage. If you were to make this a sorcery, you could probably cost it at

{W}{U}.