Don't know if this helps but I was passed this gem on editing:
If you look inside a preset file, you will see a
section labeled "ToneCurve" containing a series of numbers. You can create a
preset containing just the ToneCurve and whatever set of numbers you'd like
provided they are in the range 0 to 255.
For example, here is the default medium contrast curve:
ToneCurve = {
0, 0,
32, 22,
64, 56,
128, 128,
192, 196,
255, 255
},
The numbers are successive pairs of input and output values.
Here is the content for an invert the image develop preset file:
s = {
title = "Invert",
type = "Develop",
value = {
settings = {
ToneCurve = {
0, 255,
255, 0
},
ToneCurveName = "Invert"
},
uuid = "invert-the-image",
},
version = 0,
}
The uuid field just needs to be a unique string. It gets used to track the
last preset applied to a photo.