Kind of a late reply, but you mean the "9xx" effect command, right? Even though I'm just getting started with using MilkyTracker, i think i might have figured out a method for this type of thing:
In the sample editor, there is an option to export the wave to a .wav file. From there, if I import the wave file into Audacity, I'm able to better look at the wave than I would in the sample editor, since Audacity allows you to, for instance, zoom in.
At the bottom of the audacity window, there should be numbers that display the range you have currently selected, in milliseconds. If you click on them, it gives you the option to change it to a variety of different formats. Crucially for you, one of the options is "samples".
From there, change the first number of the selection to 256 * whatever number, which you can just put through a calculator. Play back the wave with different multiples of 256 until you find one that sounds good. Then, convert the value you settled on into hexadecimal (there are conversion calculators online), and put that into the 9xx command.
If you've already found a position that sounds good, but isn't a multiple of 256, put the starting position of the selection into a calculator, and divide it by 256. In the 9xx command, try using the resulting number rounded down, then rounded up, and see which one sounds better.
Again, i'm new to the whole tracker thing, so i hope this works for you. And, uhh, sorry for the long post