I put a low health part on my kunai for my spy. I was wondering how low my health needs to be in order to activate it. Also in general, what is the number for other classes too? Is it a number or a percentage of your health?
According to the TF2 Wiki, you would need less than 10% of your maximum health.
This is a bit hard to link, but if you go to the 'Strange Part' page on the wiki, and mouse-over 'Strange Part: Low-health kills', under the 'List of strange parts' section, you would find the answer in the tool-tip that appears.