The top 1% own 50% of the world's wealth - how do we turn this into a function? This Oxfam report states that 1% of the world's richest own 50% of the wealth. 
But to be in the top 1% - you don't have to be a billionaire (assuming a billion is US dollar one thousand million). You only need to have $US 798K in assets. (ie owning a house in San Fran, London or Sydney). 
It follows logically that the next question asked (apart from what on earth are we doing about this desperate income inequality) - what if you have US$100K in assets? Are you in the 2% or the 3% band? It's almost like you need a function to answer this question. 
My question is: The top 1% own 50% of the world's wealth - how do we turn this into a function?
 A: First you need to know what shall be the definition of the function your are looking for. Because you gave us the data points 1% of population have 50% of assets one can think about the function $$f:[0,1]\rightarrow[0,1]$$ with the definition "taking the richest $x$ of the population $f(x)$ gives the ratio of assets they have". You also asked the question

what if you have US$100K in assets? Are you in the 2% or the 3% band? It's almost like you need a function to answer this question.

For answering this question you need a function $$g:\mathbb R \rightarrow [0,1]$$ with the definition "having an amount $x$ of assets $g(x)$ is the smallest part of the richest population you are part of". Note, that you cannot deduce the function $g$ by knowing $f$! So what function you are looking for?
Another problem: You only gave one argument - function value pair. It is not possible to deduce a function by knowing only one point of its graph. For this you need to assume over properties of the function. For example one can assume, that $f$ has the form $f(x) = 1 - x^n$ which I really doubt because I do not see any reason for this.
I recommend you to look for the density function how the assets are distributed (data which says, how many people have an amount $x$ as assets). From this data you can deduce any function you want.
