What is the calculable effect of counterfeiting on an economy? I'm curious whether one can numerically calculate the effect that counterfeiting has on an economy.

As I understand it, counterfeiting essentially amounts to theft of the wealth of everybody holding units of that currency. For example, say you have an economy with 100 units of currency currently circulating. Bob creates 100 fake units of currency. If he does nothing but leave them in his safe, then the economy is unaffected. However, if he spends all of them, he will get goods and services in exchange for nothing of value. This is the theft. Him introducing his 100 fake units into the economy doubles the money supply, which will eventually more or less lead to a doubling in the price of everything (but not necessarily). 

So now, if Dave had 10 units of currency his purchasing power was X. However, after the counterfeiting and doubling of the money supply and thus more or less doubling of prices, his purchasing power is X/2. Likewise for anybody who was holding that currency.

So, is it correct to say that in an economy with X currency units, counterfeiting and spending Y fake units is equal to theft of Y/(X+Y) of the wealth of the economy?

e.g. Given 100 legit units and counterfeiting and spending 200 units, 2/3rds of the wealth was stolen?

If not then what is the effect?
I'm adding this as a separate answer because it's quite independent of my other answer.

We've agreed that if there are X dollars existing and you counterfeit Y dollars, then you've stolen Y/(X+Y) of the value of those X dollars.  

This assumes you do your counterfeiting all at once, and without warning. 

If instead you do your counterfeiting slowly over time, or if it can be anticipated, then there's an additional effect:  people try to rid themselves of money holdings in order to avoid being stolen from.  (That is, they try to trade dollars for goods before the dollars lose value.)

This is costly to the moneyholders, with no offsetting benefit to anyone.  One way to see that it's costly to moneyholders is that they now own less money, which makes their lives less convenient.  A different way to see that it's costly to the moneyholders is that the rush to trade money for goods bids up prices, which lowers the value of money.  (These are not two different costs; they are two different ways to think about the same cost.)

So in this case, you have stolen more than Y/(X+Y) of the value of originally existing money, but kept somewhat less (because by the time you get to spend your dollars, they're worth less than dollars were worth at the outset).  The difference between what you steal and what you keep simply dissipates.