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.