What you need is a spring "gizzy". I probably have a dozen that I have made up for different size springs over the years. Here is a video from the Midland series with Jack Rowe. About 11 minutes in he shows his "gizzy". Anyone dealing with doubles will find these videos a real treasure. Best regards, Jack P.S. if the link doesn't work search "gunsmithing the british double".

https://www.youtube.com/watch?v=ioxvszv2058