[bojan]

Actually, in Newtonian telescope case, the prime focus is where it would have been if there were no secondary mirror.. so you are right to the certain degree (you missed by the radius of the tube..).
But since the secondary mirror does not change the optical properties of the system (apart from moving the focus point outside the tube and flipping sides of the image) , Mike is also right :-)
So, both of you are right if we assume we are talking about Newtonians :-)
And, if we want to be really correct in naming things here, this should be called "Newtonian focus"...