Spring length for split bones

I'm thinking of going with a behind the axle spring connected to the wishbones. The axle is a stock '40, what length spring do I need?
TIA

 

Go to Posies site, or call them.

 

Yea, I e-mailed them last week. Still patiently waiting for a catalog... I was going to get a spring from Jerry Gresham at Component's Corner here in town to mark it up with but he had just sold his last one.
Thought someone on the HAMB might have done one of these before and could give me a "heads up".

 

Most that I have seen use stock model a springs.

Here's mine, stock A spring, 40 axle and bones.

 

I believe the correct spring lengthwould be 4 inches shorter than the distance between your perches....this is to put the shackles in the correct position...AM I right guys....