roar takes a flat 30%, but sacrifice deals damage proportionally to how much damage you do. if you do 20k with 10% cost, you take 2k for example. at 30k, roar takes 9k hp. sacrifice dealing 3k or so is ideal for me, but will vary individually. Note that you do not control whether it slashes or stabs, so you do not have perfect control over it. With it at 29, I have hit near 200k on apple with crit when i accidentally use it in zerk range. This deals up to 10k self damage which can absolutely kill you. It is good to not leave it too low which is why i say 20-ish