There are a few things to consider:
1. Assuming that you are using a correctly cut armboard, the distance between the platter spindle and the centre of the pivot (eg the hole in the damping knob) should be 214mm.
2. Are you using the original Jelco mounting collar, or one of my improved versions? If you have the standard collar, be aware that it does not always hold the arm pillar vertically - it tends to fall over to the left as the grub screw is tightened. This can cause azimuth errors. My Mk2 improved collar prevents that from happening (sorry for the blatant plug, but my collar really does aid easy and consistent arm setup).
3. Set the stylus overhang correctly, using an alignment tool such as those from Clearaudio or Dr Feickert, or print off a suitable alignment template from Vinyl Engine. The template provided with the HiFi News Test Record is pretty good too. Note that the old Technics plastic headshell alignment clip is not suitable for use with the Jelco.
4. Align the cartridge laterally in the headshell using a suitable alignment template. The stylus cantilever should be aligned to the null point grid lines.
5. I find that VTA is best set by using a small circular bubble level, placed on the headshell. Set the correct tracking force with the bubble level in place, and then set the arm to level with the stylus on a record. Also check that the headshell azimuth is correct (eg level) and does not vary as the arm traverses the record (if that happens, the arm pillar is not vertical in its collar, or the collar is not mounted level to the platter). When all is level, remove the bubble level and re-set the tracking force to the correct level.
6. Accurate setting of VTA depends on the cartridge, stylus rake angle, the records being played etc, so the initial level setting is only the start point. Experiment with raising or depressing the arm in its mount, and listening to some favourite records until you find a good compromise. Some people obsess about VTA and set it for every record, but I don't!
I hope that helps and that I haven't patronised you! I'm sure others will be along with more tips in due course.