Manage 16800 pixels in a 12x14 mosaic grid

I have a really tricky (for me) problem:

I mad a 10x10 mosaic which does exactly what I want :
display by random a 400 pictures folder in a 10x10 grid. there is an alpha mask which is composed by 10000 moving
pixels (100 pixels per picture).
When you press B:vvvv choose 6 pictures makes the others disappear, and the pixels mask let those 6 pictures revealed. press B again to repeat the process.
see the patch 10x10 pixels if you want to understand what I mean…

but now I try to make the same thing but with a 12x14 grid, wich represents 16800 pixels.
I keep 100 pixels per picture.
I can’t make the alpha mask disappear correctly to reveal pictures…
it must be a setslice issue- I don’t know how to spread values to make a 10x10 pixels grid upon my 12x14 pictures mosaic.

see the 12x14 to understand my issue. (blues pixels represents pixels which should be on picked images…)

I hope I’m clear, it was not possible for me to just give you the part of the patch which seems to be wrong…

thanks a lot for any tips !!!

10x10 which works:

12x14 which doesn’t work:

pixels 10x10 forums.v4p (63.1 kB)
pixels 12x14 forums.v4p (46.9 kB)

made finally a manual linear spread, works fine.

idreams 12x14.v4p (117.6 kB)