How Many Turns for the Torsion Bar?

Since there are several T-bars to chose from,and 50 year old bars likely wouldn't all respond the same way, I doubt anyone could make such a correlation.
My advice is to get the weight off the suspension, and run the adjusters halfway up for a baseline. Drop the car down, and unless you are on turning-plates, relax the suspension by rolling the car backwards 3.5 to 4 ft, and then return to the starting place. Measure from the lowest point on the K-frame at the center of the K, about under the engine balancer, looking for ~6 inches. Repeat as may be necessary.
Six inches is not the factory spec. It is just a measurement that I have found adequate for my driving style, on city streets. With 340 bars or less, you may require more than 6", for your application.