Sustain on champions help pls

Lets say a champion had 8000 total hp and an ability heals for 10% of their total hp and another champion had 800 total hp and they also have an ability that heals for 10% of their total hp do they both have the same amount of sustain because they both heal the same percentage of their health or does the 8000 hp champion have more sustain because he has more health meaning a bigger heal EDIT: Would buying more hp on the guy with 800 hp give him more sustain?
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