So simply lengthening one of the loops on the lighter spring would start the curve sooner but keep all else pretty much the same? Do I have this right?
With the MSD I don't know which spring is the more likely to effect the start.
I'd measure the length of both springs (inside of loops), and the perch distance.
If they are the same length, then I'd probably make the heavier spring longer.
If they're not the same length, then its math time or gut feeling.
Could take one off, feel how much resistance there is when you try to move the advance. Then swap them and see if there is more initial with the other.
In Chrysler and Motorcraft distributors the heavier spring has slack when installed. Some mallories too. So with those the heavier spring does nothing at idle and low rpm. That may not be true at all with the MSD.