I don't know the answer to your question, but my experience tells me it doesn't really matter because, to me, only negative local is really useful. Apply whatever global is needed to eliminate the problem and use local -100 with automask to "mask" the areas where it removes color that was really there in the original scene but just happened to be within the edge and color range selected in the global controls.
It seems to me that different combinations of removing global amount using negative locals and adding to global amount using positive locals will be optimal for different photos. Presumably Adobe agrees or they wouldn't have the local slider going both directions. I just don't have a good feel for the relationship between the local and global amounts yet...
I think I read Eric Chan discussing this as follows:
if you need more "oomph" than the local +100 gives you, then you need to go global to +20 if need be, while "protecting" areas of the image that you don't want to have CA applied to, by applying local adjustments at -100 (this protects these parts of the image from the global slider setting)
Here is the section which answers my question:
Description of Local Defringe Control
The global Defringe control is sufficient in many cases, but sometimes local refinement is required. One reason is the need to “protect” certain scene colors (prevent them from being defringed). Another reason is to help suppress some minor residual fringing in aparticular area. For these reasons, Defringe is also available as a local adjustment.
- Available as a brush or gradient (as with all our local adjustment channels).
- Only available in PV 2012.
- Standard range is -100 to +100, default 0.
- Minus direction (towards -100) means “do not apply defringe to the affected area.” This is a way for the user to “protect” certain image areas from being incorrectly defringed. For example, applying a strong purple fringe removal may indeed effectively remove those fringes, but it may also desaturate or otherwise (undesirably) alter edges of purple objects in your picture. Painting with Defringe -100 over those areas will completely protect them and keep them at their original color.
- Positive direction (towards +100) means “apply additional defringing to the affected area.” This is a way for the customer to fine-tune and take care of small problem areas.
- For images that have only limited color fringe problems in a specific area, it may actually be easier (both faster and safer) to use the local Defringe control.
- Note that local +Defringe will remove fringes of all colors (not just purple and green) and hence is independent of the global Purple Hue and Green Hue settings.
- The maximum strength of local +Defringe is limited (not nearly as strong as global defringe), so for extreme cases you will need to use the global Defringe instead. (In general, I recommend using global Defringe first anyways, then following up local Defringe if needed.)
-1 to -100 masks some to all of global purple+green defringing.
+1 to +100 defringes "indescriminately" (is completely independent of global purple and green defringe settings).
My new experience:
I've found both masking the global with a negative local and defringing with a positive local to be very useful.