http://www.geocities.com/mathfair200...trig/trig0.htm

With the original bar you know the dimensions of a right triangle that is made from the middle of the sensor bar to where you site and the middle of the sensor bar to one end. Using this information and cosine you can find the length of the third side of the triangle. With that you can find the angle of longest side (from IR emitter to person) and shortest side (from IR emitter to center of bar).

Now you have to create a triangle with the same three angles but with a 10 foot side which will tell you the length of the distance between the middle of the bar and your new IR source.