That method works on Hard too? I've only done a medium run thus far.
Medium -> 2 deaths grants a PK. Works for < 9 PKs
Hard -> 5 deaths grants a PK. Works for < 2 PKs
This also takes your existing PKs into consideration on both settings. So, for example, if you die twice on Hard and have 2 or more PKs, you won't get one.
It's on the Rockstar Newswire somewhere if you're looking for a source. They mention the difficulty levels and the above bit as their way of dynamically adjusting the difficulty.
Also keep in mind that PKs when used manually restore more health than when used automatically in Last Stand.