Duck -
My thinking is that if you use a set speed motor, say at 100 rpm, and use a 5:1 gear ratio you get a nice even 500 twists per minute. If you incorporate a timer, you can set it for X minutes and Y seconds to get the number of twists you want. For example, on the jig I have been using with a 3:1 gear ratio, the Danville 210 Flymaster Plus is best twisted 1350 times, or 450 cranks ( 450 X 3 = 1350 ). With a 5:1 ratio, set the timer at 2 min 42 sec ( 162 sec or 2.7 min X 500 = 1350 ) and just hit the switch.
To furl the same material "mechanically", I join the legs on one hook and countertwist ( furl ) 750. With a 100 rpm motor at 5:1 ratio, set the timer for 1.5 min ( 90 sec or 1.5 min X 500 = 750 ) and hit the switch one more time.
With this type of set up, you could go from measuring the best reduction ( the "I'll probably stick to pretty much Denny's way...incidently it's twist to just before breakage..." approach you cited in your earlier comments ), to determining the time it takes to reach that reduction on a given material and then use the timer rather than the reduction factor, to achieve consistent results.
Lots of good ideas here on building the new jig. Just need to figure out which ones will best work with the approach I prefer for furling leaders.
John