CUSTOM ITEM MENU - CHANGING PAGES (RPG2K3)

I'm struggling with implementing a custom menu, specifically lists of items.
I've been working after this tutorial - http://rpgmaker.net/tutorials/410/

Basically, everything works, but switching pages. Mainly, I can't figure it out because I can't make out what exactly certain variables mean, which are involved into page changes.

Here is the beginning and end of my MakeList:




Don't bother with the numbers inside the branches. The -31 and such where merely assumption based, but didn't work (as somany other combinations). The difference between my menu and the menu in the tutorial:

I have 38 spots for items and display 10 items per page.
I didn't understand why ptrA and itemlistPtr were set to 41, even less why at the end you set ptrA 50 Less/Equal.

Also here is the code I use as key input for the page changing:


So, while switching left works once, further switches will result in total chaos (empty pages, pages showing the same item twice, random items somewhere in the list etc.)

While pressing right will usually show an empty page or doesn't allow switching.
If this is too much, can someone try to explain at least what those variables mentioned above do? Maybe I can figure out what I need to do by myself then.