This is aspect of hp / mana gain should be made a standard as an option in config to choose whether you would want percentage or non-percentage values since there have been many request for this over the years.Following thread works just fine for me, you have to replace some lines uint16_t to int32_t and msg->put to msg.add.. I think it was all, long time ago I did it.
https://otland.net/threads/jak-wysylac-hp-i-mane-w-procentach.181560/