http://info.efidynotuning.com/fuel101.htm#ahisl although not totally relevant to our fords some good info in there.
Injector Low Slope
The Injector Low Slope accounts for non-linearities at low pulsewidths, at low pulsewidths an injector acts as a larger injector spraying more fuel, so to compensate you will have a low slope with a value HIGHER than the high slope. Typically the low slope is approximately 15% larger than the high slope. So a 30lb injector would typically have a low slope of (30*1.15=) 34.5, a 60lb injector would have a low slope of (60*1.15=) 69 Now the further apart the low slope is from the high slope the more influence the breakpoint will have, thus IT IS IDEAL AND HIGHLY RECOMMENDED to have the low slope AS CLOSE AS POSSIBLE to the high slope, this will insure your fueling is not doing anything odd at the breakpoint. However, the low slope MUST NEVER be less than the high slope but can be equal to.