Fmx - sliding menu AS question

Hi,
I have a few q’s about the sliding menu tutorial by Ilyas…
http://www.kirupa.com/developer/flash5/slidingmenu.htm

I’ve done a search, and found a few good replies, but i still don’t completely understand the code well enough to be able to modify it for more clips, or different sizes…

ok, so AS on the clipgeneral mc:
onClipEvent (load) {
_root.xnew = _root.mask2._x + (4-1)*100/2 ;
}

So, (i think…) that the *100 refers to the clip size (of each of the images) so if my images were, say, 300px wide, i’d change this # to 300?
And, after reading a few posts on the tute, the (4-1) refers to the number of clips total? (so, if there were 10, it’d be 10-1…)

In the buttons, this second line is also used, but the (4-1) changes as you move through the images: 4 decreases by 2 each clip. Would this also be true if i had 10 clips? (10-1, 8-1, 6-1,…)
Why? I don’t understand the logic here…
I guess what I’m asking is for someone to explain this line of code in relation to # of clips and width of clips.
Thanks a bunch!!