This is such a timely thread! I just finished assembling a Brown Bess lock for one of our own subscribers, and naturally, there was some spring alteration that was required.
Here's a picture of the frizzen spring for this lock as received and as cast, alongside a second one that is finished and fit to the lock. You can see that the spring needed to be re-arched on the top leaf and spread so that it engaged the toe of the frizzen with some tension.
How much tension is needed? Here's a picture of the spring set on the plate but not screwed down, and it shows the amount of compression required to get the screw into it. This is with the frizzen in the open position.
...and here's one the same but with the frizzen closed. You can see that the spring is under more compression when the frizzen is closed, than when it is open.
Here's one of the spring fully installed.
And just for the heck of it, here's the whole lock.