It would be 1000(x-2.5)/(1.1)

You get there by assuming there's a linear relationship between the bonus and the performance rating. If we call those y and x and if you know two points (x,y)=(a,b) and (c,d) then the formula for the linear relationship is

y = ((d-b)/(c-a))(x-a) + b

In your case you have two points: (a,b)=(2.5,0) and (c,d)=(3.6,1000) so your equation is

y = ((1000-0)/(3.6-2.5))(x-2.5) + 0 = 1000(x-2.5)/(1.1)