where to install a tonearm

Hi, Until today I always thought that all that I need to set up a tonearm is to use a protractor to find the "pivot to splidle" distance and that's it. If I want I can draw an arc with this information and I can install it anywhere in that arc.
I have a Lustre GST-801 to install and in the manual found that besides showing you the correct distances, they include 2 more measurements: they draw a line that goes from the center pivot up to the point when it connects with the extension of the horizontal line of the spindle (basically where both lines meet in a 90 degrees angle). in the Lustre, those distances 199mm (horizontal) and 105mm (vertical). If I take this into consideration the arm will be too close to the platter (my TT has a peripherial ring) and that's not practical for me.
knowing that a platter is round I always though that we can install the arm anywhere in the arc line that I mentioned before ... am I correct?, am I wrong?, please help me with this, Im really confused.