Example code:
x as float = 0.0
y as integer = 0
while x < 599.9
x = curvevalue(600.0, x, 4.0)
ink rgb(255,0,0),0
line x, 0, x, 480
inc y, 14
ink rgb(0,0,255),0
text x, y, str$(x)
endwhile
wait key
Simply put, the first number that you provide is your destination value, the second is your current value, the third is the number of step to take between these two values.
So in the example code we feed in (600, 0, 4) and it gives us a value of 150, which is 1 fourth of the way between the destination and current positions.
The next loop we feed in (600, 150, 4) and it gives us a value of 262.5, which is 1 fourth of the way between the destination and current positions ... repeat until you get to the target position
I'm not too sure I explained that very clearly. Let me know if you need anything cleared up