From the OS map, St Ninians is bang on the 70m contour and the bottom is between 30m and 40m, if that's any help. The 40m contour is just above Port-e-Chee Avenue, which is the last turn on the right before the crossroads at the bottom.

Sat-nav GPS isn't always very accurate for elevation. When I take mine on the ferry, the trip log shows some surprising slopes on the Irish Sea!

The steepest part appears to be between the 60m and 50m contours, which measures around 80m horizontally, so a 1/8 slope.