measuring 8 different (new) springs some time ago I noticed that the values from the 3 different brands were of by at least 2-3 pounds.
So spring with a certain compressed length and advertised as 16 pound springs would give the same value as springs advertised as 13 pounds at that same length.
I have had springs with the same value from the same manufacturer that were 2 pounds of.
I personally could not find any logical correlation between the advertised values, the actual value and the installed height when springs were mounted in the gun.
I'm talking about hammer springs here but I have no reason to believe mainsprings are any different (better)
I wished manufactures (aftermarket springs and guns) would state how they rate their springs. That wouldn't be to hard as this is standard for (f.i.) valve springs.
With a hammer spring I can measure changes, with a main spring it's trail and error.
In shooting we are talking about grains, grams and 1/100th of millimeters that could make a difference but with springs we are left in the dark.