Clampfit makes it easy to meassure the slope of the fEPSP.
1/ mark the part of you signal as shown of pic 1 with two cursors separated by about 1 ms (you might need more or less depending on your signal), but you need to choose a relatively linear part of the signal (see zoomed signal)
2/ go ANALYSE --> STATISTICS and just follow Pic 2
3/ click OK --> value is reported is RESUTS sheet (see Pic)