WinLTP has three methods of measuring slope: 1) Maximum Slope, 2) the BeginTime -> EndTime which measures the slope from beginning time point and to the end time point, and 3) the Low% -> High% Peak Amplitude which measures the slope from the beginning time point (where the amplitude is say 20% of the Low% Peak Amplitude value) to the ending   time point (where the amplitude is say 80% of the High% Peak Amplitude value).

Of these, the Maximum Slope measurement is usually the least fussy and is perhaps the most reasonable to use for online acquisition.  It is partiuclarly good if there is a shift in latency for some reason.

The BeginTime -> EndTime is generally considered the 'best' method of measurement to accurately reflect the underlying synaptic conductance.  The BeginTime and EndTime should be selected to be proper and unchanged for the whole experiment.  Because of this, the BeginTime -> EndTime measurement is perhaps best used for reanalysis of the slope. And if for some reason there is a shift in latency, the BeginTime -> EndTime will be artifactually affected.

The Low% -> High% is OK when there is an EPSP to measure, but when the EPSP Peak Amplitude goes towards zero you begin to measure the slope of noise.  Because of this, this measurement is generally the least useful one.

To use the Maximum Slope, you set the MaxSlope time field ("1 ms" in the figure below) so that it is smaller than the BegTime/EndTime range ("2 to 8 ms after pulse" = 6 ms).  The 1 ms MaxSlope (marked by the red line) lies within the 6 ms BegTime/EndTime range (marked by solid vertical lines on the trace).

In the example below, the MaxSlope algorhythm starts at 2 ms after the pulse and calculates the value of a 1 ms slope every 0.1 ms until the EndTime point is reached (between 7 and 8 ms after the pulse).  The absolute largest positive or negative slope will be the slope chosen.  This is the Maximum Slope, and is plotted with the red slope line.

For more information see Section 4.11.4 in the on-line WinLTP Manual.