Go to a tangent table. Select the your cone (tangent) angle and get the tangent of the cone angle. Multiply this number (tangent) by the depth. This calculation will render viewable area diameter.
That was easy.
Yep. I know. It is an isosceles triangle. But it's pretty close to the right answer.
tangent table link. Check it out.
http://www.science-projects.com/TangentTable.htm