This is an interesting subject. In the 60s when I was a teenager I used to play around a lot with Ford distributor advances. I kept a container of various Ford advance springs, and just threw them out a couple of years ago, thinking I would never need them again.
Anyway, as I remember the light one was tight and returned the advance to zero. The second heavier one had play in it until the first lighter one advanced until the slop was all taken up in the second. You could buy after market springs that both had tension on them from idle on up. There is correlation between the springs and the weights. So, some springs that work in one distributor may not work in another. You might check Accell springs. They might be close.
You may have to try several springs before you get it right. "Right" meaning a smooth advance to about 2500-3000 rpms. What I used to do is graph it out. So, if you have say 3,000 rpms that you are shooting for, and you have 20 deg. in the distributor, and your idle is 1,000 rpms, then you would want that advance to start just above your idle range and be equal through that 1000-3000 rpms. That 2000 rpm range would be divided up by that 20 deg. or 5 deg. for every 500 rpms. It was easier to set these up on a Sun Distributor machine, but you can do it on the car if you have advance makings on your crank damper. It's easier to do with someone helping with the throttle, pausing at every 250 rpms and calling it out to you while you are watching the timing marks. Them you can see exactly what you have.
If your springs are to light the distributor will be advancing at idle. That throws you off. That's one reason to check your advance at idle, set at zero, and see if you hit 20 deg for full advance. If you are only hitting like 17, then it could be advancing 3 deg. at your idle. It's hard to get it all right on, so shoot for "in the ballpark." It can be time consuming but satisfying when you get it right.
Wayne
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