Continental Quarter Repeater repeater spring replacement

[Watchfixr]

I am working on a verge fusee, either Swiss or French, automaton quarter repeater. The repeater spring is broken. The spring is probably not original to the watch. It appears to be from small pocket watch approximately a 6S as best i can tell. I need to install a new spring and am trying to determine the strength and length of the new one, The rack arbor diameter is 2.85 mm and the barrel diameter is 8.28 mm. A spring width of 2.0 mm will fit the barrel. The arbor turns less than a full revolution when fully wound for a 12 hour strike, so the spring doesn't have to be a long one like those used in a going barrel. The barrel can be adjusted for preload by a click on the barrel teeth. I would think the preload is only to remove slack from the spring to keep the chain tight.

The dilemma is how to arrive at a length and thickness that will allow for full action of the repeater and provide for repeated use over time. The speed regulator is a train of wheels terminating at a tilting pinion, common to these watches. So, there is a range of adjustment.

I do not have another watch to examine for the proper spring.

Any help is greatly appreciated. Trial and error is always a way to arrive at a solution, but it is time consuming and expensive.

thanks in advance.