"adjust till you get more top speed."
I have an Echo 2000 and I see the 2 nuts that lawnboy dan is talking about.
I think this is the only adjustment for the cable. I don't think there is another adjust ment at the trigger end.
The length of the cable really does not matter, if it is a .25 inches too short or too long, it makes no difference. It is the positon of the cable housing, in the clamp that these 2 nuts adjusts the cable housing within, that matters.
I do NOT think you should be adjusting the throttle cable using top speed as what you look at. That could cause problems with idle speed. You could easily get things so that the throttle never closes enough at idle speed, and then it will be impossible to adjust the idle properly, with the idle adjustment screw. You must adjust the cable by watching the throttle fully close, and then loosening the cable a bit more, so that you pull the throttle trigger about 1/8 inch, before you see the throttle plate cam move at all. You might even want to turn the idle adjustment speed screw all the way out, before adjusting the cable, making sure there is play in the cable. Then turn the idle speed screw back in, to adjust the idle to the correct speed. Preferabley with a tachometer, but you could eyeball it by the way the centrifugal clutch stops the trimmer head from turning -- it stops it from turning at a bit above idle speed, if it is working correctly -- so you adj the idle speed until the head stops turning, and then lower it still a bit more.
So if you adjust the play at closed throttle, then the max throttle posit should automatically be correct. That is because the throttle cam, and the trigger, both move a fixed distance, which is non adjustable. The throttle cam will hit the carb stop plate, just about when the throttle trigger is all the way down. If the cable, and the trigger, does not have way way too much play in at at idle, then the throttle will open all the way, at, or before, the trigger is fully depressed.
You could check to see that the throttle cam is hitting the carb stop. If it is, the is no point in adjusting the nuts to shorten pull the cable housing up further. The throttle simply won't open any more. Also, if the cable is too tight, and the throttle cam hits the carb stop plate before the throttle trigger is fully depressed or nearly fully depressed, it could make things so that during use, the throttle trigger eventually becomes damaged, and also the cable stretches faster over the months, than it should.