To calculate the center of the Ripper's pentagram, I calculated the slope of the line between the Mitre Square location and the Durward Street location, and the slope of the line between the Henriques Street location and the location of the upper midpoint, i.e. the midpoint between the Dorset Street location and the Hanbury Street location, using the usual bearing-calculator on the Movable Type Scripts page of map-related calculators. Then, using the calculator for calculating the intersection of two lines, given the start points and the slopes, which is also located on the aforementioned web-page, I calculated the intersection, which is the center of the pentagram, and obtained 51.516667, -0.069722. (The map has been updated with these new coordinates.) Compare this to the previous coordinates, 51.516687°, -0.069764°, which I determined by drawing these same lines in Google Earth and placing a push-pin at their intersection. The fact that the results are so close to each other indicates that the formulas used for these calculations are accurate.
I then calculated the bearing of the pentagram-center relative to St. Paul Cathedral's dome, and obtained 8.944167 degrees. This corresponds precisely to the sunrise on August 30th, 1888, which is the same date that the previous pentagram-center-coordinates yielded, and which was just one day before the Ripper's reign of terror began.