Lately I've been learning about the marvellous beauty of of Bullet. Mostly have the bullet disabled and turn of angle to motion.
Bullet.set angle of motion angle(bullet,player)
Wait x seconds(where as X would be timed to 30px)
determine how much time you want it to take to travel 30px, then figure how much the bulletspeed should be based on that.
As an example. If you want it to take 1 second to travel 30px
Bulletspeed = 30
WaitTime = 1
30px over half a second
bullet speed 60px
wait time 0.5
boom there you go. 30px