Please, PLEASE do not give Singed a % max health damage
http://www.surrenderat20.net/2015/03/313-pbe-update.html#more
Singed
- Fling (E) damage lowered to 50/65/80/95/110 from 80/125/170/215.
- Fling (E) now also deals 6/6.5/7/7.5/8% of targets Maximum health (max 300 bonus damage against minions and monsters)
Of all things you can do for Singed, this is not even close to what you should do.
Sure, it will make him a better jungler, but there is a 99% chance that it will become a problem later on, resulting in the rest of his kit to be nerfed. % health damage is extremely hard to balance (see: Vayne, Nidalee, Elise, etc), so it should be avoided at all possibilities. I do not want to see an original champion be changed and nerfed because of something as stupid as this being put into his kit.
Guinsoo would never allow something like this to change on his champion.
(I know it is in PBE, but might as well try to talk them out of it early)