how to calculate percentage of chance in a price negotiation I would like to know how to calculate a Percentage of chance in a negotiation. 
Here is the problem : 
The product has a price of 30€ which represents an acceptance rate of 100%
The minimum price accepted is 27€ which represents an acceptance rate of 50%
How to calculate the acceptance % of the prices between 30 and 27 ? 
Thank you for your help.

First we have to assume a linear relation between price (p) and acceptance rate (a). Next we calculate how much on average the  acceptance loss is if the price is increased by one pound.
The difference between 30 pound and 27 pound is 3 pound. The difference between the corresponding acceptance rates is 50.
Therefore $\frac{50}{3}$ is the loss of the accptance rate per pound. In total the acceptance function is  
$a(p)=100-\frac{50}{3}\cdot (30-p)$ for $27\leq p\leq 30$
If $p=29$ the brackets become 1. By adding the brackets we get the price difference.
