So I noticed the other day that the pull cables on my extension spring
garage door had fallen off of the pulleys (the ones vertically aligned
with the door when its down) on both sides.

I decided to start by replacing the cables. The pulleys all seem fine
- they turn and are not bent in any way. I did this on one side, and
I noticed the instructions sort of breezed over how tight you should
make it (ie where to secure the other end of the pull string to the
fixed position on the ceiling). I basically placed it such that, when
the door was open, the cable would remain just taunt between the
pulleys so that they would not sag.

I then took a closer look at the side which I had yet to replace. I
noticed that, when the door is fully opened, the wire sags severely
between the pulleys. There is no chance it would stay on without any
tension. Here are my questions:
1) While working, I had the garage door fully pulled up so that it was
completely flat in the opened positoin (and braced with clamps). I
can't remember for sure, but I assume that in normal operation it
never goes up that far - it must stay down a bit where the last
segment actually bends slightly. If this is the case, I think the
cable has a better chance of staying in the pullys, but right now I
don't even think my newly installed side would stay this way. So,
does the pully system rely on the door never being completely
horizontal?

2) Does the sagging indicate that my springs need to be replaced?
They still seem to pull up the door when the pully remains on the
track, so I assume they are OK.

3) What is the proper method for setting the tension of the pull cable
in a resting position? I'm concerned about it being too loose and
causing the cable to fall off the pulleys in the up position, or it
being too tight and causing breakage of the springs in the down
position.

Thanks,
jeff