Need to make a thread dial - need the basic concept

Working on my long-term seneca falls lathe project, and want to build a thread dial for it. it's got a 12 tpi leadscrew, and I'm limited in the diameter of the gear bacause of the carriage layout (halfnuts lever protrudes from the right side of the carriage). Years ago some kind soul stepped me through the plannning for it, but time (kid, work, grandkid, etc intervenened. Did manage to paint the machine though!

Any suggestions greatly appreciated.