You're going to have to pick a maximum value, such as 100kbs then set the rate based on the percentage.
long bps = (long)(102400 * ((double)percent / 100.0);
if (percent == 100)
bps = ThrottledStream.Infinite;
ts = new ThrottledStream(originalDestinationStream, bps);
I haven't compiled it, but I would guess something like that.