Well, it would have been nice to have some sort of drawing just to make sure one has a clue what you are doing. If my drawing is anywhere near, then it looks like you have it. Why do you doubt?
How far from the door must a ramp begin in order to rise 30 centimetres with an 8 degree angle of elevation?
tan 8⁰ = 30 / x
30 ÷ tan 8⁰ = x
213.4610917 cm = x therefore the ramp must begin 213.4610917 cm from door
Is this correct???