Reducing numbers of points.

Hello,

I'm trying to do this but without succes, i dont really find how to do:

I generate a rectangle, offset it several times to have got superposing surfaces, and then i populate points on them. What I am trying to do is to reduce the numbers of points on each surface, but the higher we are going, higher is the reduction. 

For exemple: I have 10 horizontals surfaces. The 1st (z=0) hase 100 points, the second (z=2) 80 points, the third (z=4) 70 points...

I tried with coordinates without succees, actually I'm trying with %.

If someone could help me it would very nice !

Thanks :)