In the section Mounting the bushes in the instructions it is described how the position of the 20 mm (13/16 ) hole is found. For 12 armtubes the distance from the center of the hole to the center of the turntable patter should be 294.1 mm (11 9/16).
Overhang of the stylus should be 13.3 mm (17/32). If the arm bush is mounted exactly at above distance from the center of the turntable platter, the stylus should be positioned just under the front edge of the black plane of the arm tube.
Please note that the combination of 12 blue armtubes and relatively heavy cartridges may require an additional large counterweight for balancing. This will increase the effective mass, and use of the 12 red armtube instead should be considered.