As far as i know your raise HAS to be double the last raise, so if its raised to 300 you cant make it 500 it has to be 600 or more (n/l). (unless your all-in)
Now heres where it gets interesting, if you open the betting first with a raise to 300, and a player raises to 599 because he/she is all in, the betting round does not re-open. Frustrating when you pick up aces utg and raise only for utg+1 to raise all-in for less that the min, and get 2-3 callers, you cannot re-reraise, because the betting wasnt re-opened.
Get it ? It took me a little while to realise this one as well.