vertical scrolling by a constant, not seconds

I've had to switch from simple marquee code as my scrolling function to this since it's no longer supported by the software I'm using. The big problem is that I can seemingly only scroll the text by seconds until finished scrolling. As I understand it, animation: only works with Xs for scrolling. That means always having to manually checking if the scrolling is correct. Is there any way to have it scroll based on an adjustable constant?

To answer the inevitable question: Yes I know scrolling text is not good for general application. This is a small private project for a group of friends and the scrolling text will be filled with jokes, it is not important for the user to actually pay attention to it. The text will be way too long to have it static.

Edited for clearity