Let me preface this post by saying that I am somewhat challanged mathmatically. I am however fairly adept at problem solving through process, but quite frankly when high school introduced me to algebra, I went to sleep.  Anyhoo......   I'm working with Ron Barnett's formula raytrace algorithm, specifically the toroidal spiral function. The params I'm specifically working on concern the xmax, xmin, xstep, and xradius, which are arrayed in the global section. I've added a param "slide" and given it a default value. Arrays in the global section read as such, and are separate arrays:
radius=@radius
xlimit=round((@xmax-@xmin)/@xstep)

 xmax determines the maximum length of the "rope" numerically, and vice versa for the minimum.
radius determines the size of the "balls" that are strung together to form the "rope"
step determines the numerical distance between the "balls" within the xmax-xmin range.

What I want to do, is tell the argument that I want the radius to be reduced, according to my @slide parameter, progressively at each increment moving away from xmax until xmin is reached.
 My attempted statement reads:  radius= radius-(@slide*x)  where "x" is an integer on a sliding scale based on the xlimit increment position as it relates to xmax and xmin.
  My chief problem is that I don't know how to tell the argument (@"x")to recognize the increment position rather than it's numerical value.

                                                  Any help?

99.9% + 10% = 109.9% > 100%

I can see quite clearly that English is not a language you understand very well !!     

First of all, I was discussing percentages as the primary subject.  The prepositional phrase "of the world's population" was only a qualifier, and not the actual numeric quantities being discussed.

So....  the "other 1/10th" was referring to a single percentage.

If you need assistance again with English, and/or with your math, please do not hesitate to speak up.