Assassin Mastery Range? Not Specific Enough!

What is the Range at which you have to be at for "no allied champions nearby"?

Is it AA range?

's Isolation range?

's AA range?

's Passive range?

ult range?

"No allied champions nearby" isn't very specific. What is the actually range on this?

Could I be on blue side mid lane, get tower dove and not get this bonus cause my allied jungler is at chickens?

Do towers count as allies as they do with kha's isolation?

Is it near me? or near my target? or both?

Could I play , ult from mid lane with "no allied champion nearby" onto a target that is currently "nearby" one of my allies in bot lane and still proc the bonus damage?

So many questions, and such little time!