The only spring winding I have done was on a lathe. The arbor determines the diameter and the variable is the feed rate (pitch) of the spring. On a lathe, this is determined by the carriage feed. I would guess that the Hjorth uses an adjustable gauge for this.
There is another variable to consider, which is the start and stop areas where the coils lay tight against each other. On a lathe you simply feed that area by hand before engaging the carriage feed, then disengage the feed near the end and finish that end by hand. On the Hjorth, this should involve the gauge.
Spring wire will "spring back" from your arbor, so it needs to be smaller than the finished inside diameter of the spring. There will also be a similar effect on length, but it is not large. Practicing using mechanics (baling) wire will let you test the pitch of the coils and let you perfect the start and stop parts of the winding.
I always use coils of spring wire, so running out of wire prematurely isn't much of a problem. Trying to use the last bit of the coil could "bite" you, and should be avoided.
It may be easier to set up your arbor to turn, like chucked in a lathe or drill press, and have an assistant turn the spindle by hand while you focus on handling the tool. I would not turn the spindle with power unless you have a setup with complete control of speed and start/stop.
