Not completely sure if this is right, but this is what it looks like to me:
[100/150/200 (+60% AP), increased by 3% for each 1% of health lost over 4 seconds]
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Early on: [100+ (100 AP x .6)] = 160 heal if no damage has been taken in the past 4 seconds
4 seconds ago he was at 100% max health, now is at 50% max health
50% lost x 3 = 150% increase on the heal
160 + (160 x 1.5) = 400 heal total
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Late game: [200+ (400 AP * .6)] = 440 heal if no damage has been taken in the past 4 seconds
4 seconds ago he was at 50% max health, now is at 30% max health
20% lost x 3 = 60% increase on the heal
440 + (440 x .6) = ~700 heal total
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Very late game: [200+ (700 AP * .6)] = 620 heal if no damage has been taken in the past 4 seconds
4 seconds ago he was at 100% max health, now is at 10% max health
90% lost x 3 = 270% increase on the heal
620 + (620 x 2.7) = ~close to 2200 heal total
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Very late game, no AP: [200+ (0 AP * .6)] = 200 heal if no damage has been taken in the past 4 seconds
4 seconds ago he was at 100% max health, now is at 10% max health
90% lost x 3 = 270% increase on the heal
200 + (200 x 2.7) = 740 heal total