Dynamic text ticker in Adobe After Effects

[Avinash Ramanath]

Hi,
Am trying to create a dynamic text crawl/ticker. The content for the ticker is derived from a text file using an expression at source text. what is the ideal speed so that the text does not jitter. Am currently using this expression:

rate = 2; //value in px/sec.
value – [rate*timeToFrames(time),1] (i don’t even know if this is the right expression)
The ticker is for a PAL project. 720X576, 25 frames and 4:3 aspect ratio.

If the speed of the ticker is based on the above expression, then how to determine the duration of the composition considering X as the number of characters in the ticker content. Am using a fixed width font “Courier” for the content and the content can go upto 1500 characters.