I'd post this question on Audio Karma or Vinyl Engine, where there are larger DIY communities. I haven't mounted a Jelco, but have thoroughly read up on the process as it applies to Thorens tables. This is the first I've read of any discrepancies with the published length specs. However, when it comes to their effective mass, it's a mystery, no one agrees.
Whether a cartridge sits in the middle of the hesdshell is irrelevant. What is important is that it can align to both null points on a two-point protractor, or in your case, align to the grid with the arm/spindle jig in position. Carts and stylus' come in various lengths, so focusing on the middle of the headshell will only lead you to the mad house.
Whether a cartridge sits in the middle of the hesdshell is irrelevant. What is important is that it can align to both null points on a two-point protractor, or in your case, align to the grid with the arm/spindle jig in position. Carts and stylus' come in various lengths, so focusing on the middle of the headshell will only lead you to the mad house.