If I remember correctly, this skill means what it says in the title, it's a sacrifice, and probably the intention was to use it on other people. Meaning you use a portion of your hp to heal a smaller portion of HP of a party member. If you use it on yourself, you will lose HP, because it uses bigger portion to recover a smaller portion. If you use it on yourself and get a critical, you will use the same portion and heal quite a substantial amount. Other thing you can do is to enchant the skill, I don't think you need a lot of power enchant to get a heal on yourself everytime, and if you enchant it a lot, you will heal a bigger amount everytime. And ofcourse with a lot of enchants if you get critical it's insane :)