Tim, one note that you might want to remember on your lathe, it will
make you life easier.

Even though you do not have a thread chasing dial, you can still
dissengage on threds that are a multiple of the leadscrew.

For example if your lathe has 4 TPI, you can thread
4,8,12,16,20,24,28,32 etc and disengage because the lathe will follow
the same path every time you fully engage the half nut.

Your lather probably has 6 or 8 TPI leadscrew so you can figure out
multiples and save you self some time and grief.

-S