You can set
proc_mode = PROC_LATE;
in the while loop of your turret attach function to put the function at the end of the scheduler. Then it should be executed each frame after you have already moved your ship.
A more robust approach (imagine you may need to set the same proc_mode in the ship function at some point) is to call the turret placement function (without a while loop) at the end of your ship loop, then you don't need to worry about proc_mode stuff and it will always work.