Your max liftoff weight will also take into account the delay on SU motors. F42-4 will have a lower max liftoff weight than an F42-7. They can both lift the same weight, but the lighter rocket will coast longer/go higher so it can have a longer delay. Too long of a delay on a heavy rocket and it may pop the chute after it hits the ground.
I think Tripoli requires a minimum 3:1 thrust to weight but not positive. I go for around 5:1. I like slow liftoff and have put up several with an off the rail speed of 41 ft/S.
Speed at end of rod/rail is just what it sounds like. That is how fast your rocket is going when it reaches the end of the rod/rail. Generally the longer the rod/rail the faster the rocket will be going when it leaves or reaches the end. Lower thrust motors will need a longer rod/rail.
As for the thrust, you are looking at the spike right at the beginning, first .2 seconds or so. If your rocket doesn't hit the speed you need on a 4ft rail, it may possibly reach the needed speed using a 6ft rail.
One rocket I've been simming is going 35 ft/S at the end of a 6 ft rail, with a particular motor, but 42 ft/s off of an 8 ft rail, same motor.
Hope this helps
What motor are you looking at?
Mikey D