Either use the formula,

 

            Rate of descent       =          glide angle  x  groundspeed         x 6080

                                                                    60                   60

                                                =          3.25 x 140 x 6080

                                                              60      60

                                                =          768 ft/min

 

            or use 5 x groundspeed and fudge it by a factor of 3.25 ÷ 3 to compensate for the steeper than usual glideslope:

 

            5 x 140 = 700ft/min, 700 x 3.25 ÷ 3 = 758ft/min.