Determine Lens Processing Centers in Making Bifocals

A patient with refractive errors, optometry prescription is R: -4.00DS/-1.25DC*100, L: -5.00DS/-1.75DC*180, Add3.00DS, PD: 64mm.  Customers booking with a pair of flat top bifocal glasses, chose a full-frame metal frames, sub-lens height of 19mm.  How to Determine Lens Processing Centers in Making Bifocals?
Solution Guideline
1 Determining midpoint of sub-lens cutout of bifocal lens, use oil pen in the outer corners of the sub-lens cutout on each end point of a small printed dot, and then measured two feet horizontal distance dots dotted vertical line is a sub-lens basis points.
2 Produce bifocals template with blank of template, it same to the way of produce single optical glasses template, but on the template you want to set the horizontal and vertical lines. Then make a good template mirrors inlaid in the right circle.  According prescription count 60mm, dram a vertical line on template, placed the integer ruler at the center of the nose bridge, in the nasal mirror ring around the edge of the mirror as a reference to the circle draw a vertical position monocular NPD30mm. Next step, according to prescription count of 19mm, draw a horizontal line from NPD vertical line to 19mm line.
3 Set work done template on centering device machine, then put bifocals lens on template, at the same time, make sub-lens basic point overlapping with vertical line on template, horizontal cutout of sub-lens overlapping horizontal line on template, after all be done, the suction cups on bifocal lenses that  is processing center.
4 Edging, the ways like processing single optical glasses, then whole process will be finished.